XPS 15 or XPS 13

@Verttx  One more suggestion in addition to my post above: You could point out that the XPS 15 has a sister system called the Precision 5550.  They are the same system except that the Precision uses a Quadro GPU rather than a GeForce GPU as used on the XPS.  A Quadro GPU can make a difference for certain applications.  But notice that the Precision 5550 is definitely not billed as a gaming system.  It's also quite a bit more expensive thanks to the Quadro GPU.  But honestly I'd look at the X1 Extreme.

@Verttx Last potential thing you can do: Show them published recommended system specs for your CAD application, Unity, etc.  I'm pretty sure those specs will all mention some sort of NVIDIA or AMD GPU.  Then show them that the XPS 13 doesn't have one of those, while the XPS 15 and other systems discussed here all do.  And be straight with them.  Acknowledge that yes, a system with a discrete GPU will indeed be better for gaming than a system like the XPS 13 that only has an Intel GPU.  But explain that that doesn't mean that the ONLY legitimate purpose of a discrete GPU is for playing games.  It's possible for better hardware to deliver benefits in multiple different use cases.

Hopefully your parents are open to learning about why a discrete GPU matters and that it has uses other than games, because otherwise they'll essentially be saying, "We want to buy you a laptop for school as a gift, but we're going to decide what you need even though we don't really understand what you need, and we're not going to get something that we think is for gaming even if it might also be beneficial for some other use case, and we're not going to let you try to convince us otherwise." That wouldn't really be a very helpful gift offer.

@Verttx  To help explain that the XPS 15 really isn't for gaming, point out that Dell has Inspiron Gaming and Alienware product lines for that purpose, both of which are far better for gaming because they have more robust cooling systems and therefore won't throttle performance under heavy load like the XPS 15 will.  You could also point out that the XPS 15 is essentially the PC equivalent of the 15-16" MacBook Pro.  If they still won't budge, see if Lenovo has any sales on the ThinkPad X1 Extreme.  That's the XPS 15's only real competitor, and a ThinkPad is about as "business" as it gets.  I also happen to think it's a superior system for a whole bunch of reasons that I discussed in this thread.  So actually you might want to look into that anyway.

Or if they still won't budge, perhaps suggest that they cover the cost of an XPS 13 and you'll pay the difference to get the system you want and know that you need?

I really don't understand where their "too big" concern comes from.  The XPS 15 thanks to its thin bezels is the same size as a 14" business laptop, which is basically the standard size for business laptops.  But more importantly, they won't be the ones using it all the time, so why do they care about the size at all?  And for the things you're trying to run, you're not going to find a smaller laptop that performs those tasks adequately, unless maybe you got an XPS 13 plus an eGPU enclosure and desktop GPU to install into it.  But then you'll only be able to run the tasks you talked about while actually connected to the eGPU, and that solution is arguably a much more gaming-oriented setup than the XPS 15 would be.

"Hopefully your parents are open to learning about why a discrete GPU matters and that it has uses other than games, because otherwise they'll essentially be saying, "We want to buy you a laptop for school as a gift, but we're going to decide what you need even though we don't really understand what you need, and we're not going to get something that we think is for gaming even if it might also be beneficial for some other use case, and we're not going to let you try to convince us otherwise." That wouldn't really be a very helpful gift offer."

Funnily enough, I think that's what they are suggesting.

I checked my school website and it says that Unity and Adobe both need at least 2GB dedicated VRAM and 4GB recommended, I think that'll be enough to convince my parents. Plus, the XPS 15 model I'm looking at has a 6-cell 86wHr battery, which has been tested to last 25 hours on 300 nits brightness, compared to the XPS 13 which only lasts 3-5 hours on 300 nits brightness. Moreover, my school doesn't allow students to bring chargers (weird, I know), therefore the XPS 15 has the clear advantage. I don't actually game a lot, just Minecraft and maybe Microsoft Flight Simulator.

@Verttx  Just so you know, there is absolutely no way in this particular universe that an XPS 15 will deliver 25 hours of battery life while the XPS 13 would only deliver 3-5 hours in the same test.  There are a lot more variables than display brightness when it comes to battery tests.  Frankly I'd be amazed if you got the XPS 15 to deliver 25 hours under any realistic scenario, and I'd be amazed if you only got 3-5 hours out of the XPS 13 under a typical scenario.  If anything those numbers sound swapped to me.  I absolutely would not consider the XPS 15 to have a clear advantage on battery life.  You're going to want a charger with that system more than you would on an XPS 13.  If battery life is a priority, then that eGPU solution I mentioned earlier really might be worth considering as long as you can live with discrete GPU acceleration only being available while you're attached to it.  If you have any savings of your own, maybe you could put up the funds for the eGPU enclosure and the GPU.  That way your parents pay for the laptop they seem to insist is the right one, while you get the performance you need.

But I was suggesting that you look at vendor websites for recommended specs, not your school's site.  But if you really can't get them onboard for anything other than the XPS 13 and the eGPU solution wouldn't be practical for you, then honestly think about paying the difference yourself.  I can sort of understand your parents feeling that they should have some say in the laptop you buy if they're paying for it, but hopefully you can counter that by pointing out that hopefully they'd want their money to be spent as usefully as possible.  A laptop that doesn't do what you want it to do isn't going to be a great gift, which is why I suggested maybe asking them to pay for the laptop they think is appropriate and you pick up the difference.  Or if at all possible, use or accumulate the necessary amount of savings to just buy it yourself as a step toward becoming more self-sufficient now that you're going away for school (I'm presuming college here).

@Verttx  And by the way, if your school's restriction on chargers becomes an issue (what is that about, seriously?), then consider getting a power bank that's capable of delivering enough output to charge a laptop, like this one that I have.  Its 20,000 mAh capacity is about 60 Whr given the voltage of its cells, to give you an idea of how it compares to the capacity of laptop batteries.  It can deliver up to 60W output levels, compared to most power banks that typically only support up to 18-30W output.  The XPS 13 only needs 45W, so it would be completely fine for that system. The XPS 15 is designed for 130W.  It will run on 60W, although you'll experience significantly slower battery charging and throttled performance in that scenario.  But it would still be fine for basic classroom use, and you could also charge it in your backpack while not using the system I guess.  The only power bank I've seen offer higher output levels is the Lifepowr A3, but it's significantly larger and more expensive.  And you won't find 130W anywhere because that exceeds the 100W max of the official USB PD spec.  Dell did something proprietary to stretch that out to 130W for systems like the XPS 15, but they haven't made any power banks that support that.