XPS 8900, SSD 80x22mm query

I suggest you search this forum. Here is one post that may help: https://www.dell.com/community/XPS-Desktops/XPS-8900-M-2-NVMe-compatible-M-2-speed/m-p/6236401

If, as the post indicates, the M.2 slot is only PCIe x1 you may be better off with a 2.5" SSD.

Thanks for the detail and photo of the layout. My plan was just to expand the existing 32GB SSD to perhaps 128GB by simply unplugging the existing card and plugging in a substitute. The query was about physical space in the machine and getting an opinion about the practicality of this approach, unplugging cables and removing panels is non-trivial here - things are quite crowded and the machine is always busy! I'm not interested in squeezing data through bottlenecks at increased speeds, just in increasing the capacity of the 'data' drive. I think the original idea of Dell's machine was to use the SSD as the system drive and the large hard disk as a data drive. I rather reversed that to allow some programs to access data quickly. With cheaper SSD becoming available I'm just exploring the idea of reinstalling Windows onto the SSD and using the hard disk for a data store. The Windows install here is quite large with some virtualbox files taking gigabytes each. I know those can be held on a data drive rather than the system one. 

When people start throwing labels like pcie and sata and m2 about my eyes tend to glaze over!

@qwerty1357 I recommend you consider something larger than a 128GB SSD if you are planning to install Windows and virtual machines on it. I would consider 256GB and even 500GB.