Looks like m.2 2280 worked out just fine for t450s : r/thinkpad
Lenovo ThinkPad T450 i5 2.20GHz 8GB 256GB SSD 10P - IT Supply Guy
Lenovo Thinkpad T450s Hard Drive Replacement - iFixit Repair Guide
Lenovo ThinkPad T450s i5-5200U 8GB 240GB SSD 1600x900 A Class Windows 10 Professional 240GB SSD \ Windows 10 Professional | Laptops \ Manufacturer \ Lenovo \ Lenovo ThinkPad | AMSO
Refurbished: Lenovo Laptop ThinkPad T450 Intel Core i5 5th Gen 5300U (2.30 GHz) 16 GB Memory 256 GB SSD Intel HD Graphics 5500 14.0" Windows 10 Pro 64-bit Grade A - Newegg.com
Upgrading a ThinkPad T450s with an M.2 SSD - James Ridgway