30 = 30 mm, 80=80 mm.  The physical size is easy to determine visually.  You can also look up the model number of the SSD with its manufacturer. There is a recovery partition on the drive, or you can download the recovery media from Dell or Microsoft. No, there's no recovery disc (they're all but useless now since just about nothing comes with a DVD reader) -- the recovery can be done from a flash drive.  
I'm taking delivery today on a new XPS 15 9520 (7171SLV-PUS) that comes with two 256GB SSD's pre-installed for total storage of 512GB.  I'd like to upgrade to total storage of 2TB by replacing each of the SSD's with larger units.  I'm not too concerned with the physical part of swapping SSD's but I do have questions about the exact new SSD's to buy and the best time to make the swap. 1.  Per the laptop's specs, it has M.2 PCle NVMe solid-state drives.  Looking at available replacements online, I find a bewildering array of different model numbers and descriptors.  Do I need to find a replacement that says exactly M.2 PCle NVMe or are there variations I can use? 2.  I briefly watched one video about changing SSD's and it mentioned adding a heat sink when increasing the size of the SSD.  I'd appreciate some guidance there.  Is this necessary?  What should I look for? 3.  It seems to me the best time to make the swap is when the laptop comes out of the box and before I add any of my data.  Is this reasonable?  Does the Windows 11 software reside on one of the two SSD's or does it reside somewhere else?  If it's on one of the SSD's, what's the procedure for saving it and writing it back to one of the new SSD's?   Am I missing anything?  Many thanks in advance for any guidance.
Swapping SSD in New Laptop