If you’re familiar with M.2 SSDs, then you know that the standard sizes are named “M.2-2280”, “M.2-2230”, and so on. The last half of the number refers to the length of a drive’s PCB, so an “M.2-2280” SSD is 80 millimeters long. M.2 SSDs officially can be up to 110mm in length, or as short as 30mm.

The first half of the number is the width,

Source: Hot Hardware – This Specification Change Could Prevent You From Upgrading To A PCIe 5 SSD