"OS Partitions" option when building a new PowerEdge

I suspect few people know what "none" means here, as they are most likely to pick one of the other options.

None:  This could simply be the minimum partition size allowed by Windows (32GB). Or (less likely) it could simply be that Dell includes the OS media but no factory installation of the OS.

40GB Microsoft OS Partition Override:  Simple. Probably too little to be sustainable for MOST uses, but if you know what you are doing and need to streamline the C: partition, then this may not be such a big deal (the page file could be moved to another "disk" as well). I have a 2012 installation that has been sitting at around 35GB since December (including 8GB page file) - it is not a substantially utilized server, running as DC and WDS server (obviously the images are stored on a second "disk").

80GB Microsoft OS Partition Override:  Again, simple. Would I deploy a server with an 80GB C: drive? No, I'd give it at least 100GB (depending on what it's for, I usually use around 120GB). However, 80GB - if you were stuck with it - would probably be manageable (if you could move extremely large page files to another disk).

Maximum Partition Size Override for Microsoft Windows Factory Installed:  This will create a C: partition encompassing the entire size of the "disk".  If you have a 4x500GB RAID 10, with around 1TB of usable disk space, then your C: partition will be 1TB.  This will work for any "disk" configuration up to 2TB, which is why the GPT option is available.

Maximum Microsoft OS Partition Override, GPT Enabled:  Say you have a 4x2TB RAID 10, giving you around 4TB of usable disk space. In order for Windows to see and use a "disk" that large, it has to first be converted to GPT. However, Windows can only boot to a GPT "disk" if UEFI is supported and enabled on the system. This would then give you a 4TB C: partition ... I'm not 100% sure that is how it would come, but according to their description, that would be my guess.

Personally, I would select the 80GB partition. With 2008 and later, it is SIMPLE to extend and shrink partitions in Disk Management/diskpart, so I'd get it at 80GB, then extend to 120GB or whatever seemed appropriate for the server setup.

"from what I understand the unallocated space has to be "right behind" the C drive"

This is exactly right.

"So in the case of a single RAID 10 array, if the server comes with an 80GB OS partition, and then I go on to create another partition for data, I would then be unable to extend the C drive, correct? Or at least with the built in Windows tools?"

This would also be true IF you had data on the D: (or whatever) partition that you could not (or did not want to) delete.  Since that partition will be empty anyway, it is an easy thing to simply delete the D: partition (leaving 'unallocated space' in its place), extend using Disk Management (right-click C:, Extend), then recreate the D: partition.  Much easier to give yourself ample space in the beginning than trying to take some back in the future :)