Looks like m.2 2280 worked out just fine for t450s : r/thinkpad
Lenovo ThinkPad T450 i5-5200U 8GB New Drive 480GB SSD 1600x900 A Class NEW 480GB SSD \ Not included | Laptops \ Manufacturer \ Lenovo \ Lenovo ThinkPad | AMSO
Upgrading a ThinkPad T450s with an M.2 SSD - James Ridgway
Lenovo Thinkpad T450s Hard Drive Replacement - iFixit Repair Guide