Anything that physically fits into the system will work -- but be careful with the depth of the SSD.  The larger the capacity the larger the drive is likely to be physically - and there isn't that much room inside the system for a deep SSD.

Thank you

We will have to await other m15 R5 users to post their tested drive capacities. We can tell you that largest capacity SSD that Dell tested was 2TB.

Hi folks, FYI I have successfully installed a 4TB device in the 2nd SSD slot.
It's a  WD Black SN750 and it is working perfectly.

Cheers

awsome your the only human I've heard that even used a 4tb m.2 ssd in the 2nd slot. So what is in your 1st slot if you don't mind me asking.

Hi, sure, it's a 1TB drive.

Cheers

Hi @Ovenglove welcome to this free user-to-user Alienware laptop discussion forum. 

The Crucial System Advisor's database tests says Alienware m15 R5 (5/4/2021) will accept 4TB SSD cards. Alienware Alienware m15 R5 | Memory RAM & SSD Upgrades | Crucial.com. Check that the BIOS boot list UEFI option has been enabled. 

My Alienware 17 R5 boot drive is the M.2 2280 Crucial P3 Plus CT4000P3PSSD8 PCIe Gen4.0 x4 NVMe SSD with up to 4800 MB/s read, 4100 MB/s Write. Please install the Dell low profile copper M.2 2280 SSD thermal shield heatsink DP/N 26X1Y 026X1Y. Solved: m15 R5 Ryzen, SSD thermal shield - Dell Community. 

UserBenchmark application says 4TB Crucial Boot Drive has exceptional SSD score:-