Looks like m.2 2280 worked out just fine for t450s : r/thinkpad
Cha-m2b7-m256 256gb Ngff M.2 Ssd Module 256 Internal Solid State Drives With 256mb Cache For Thinkpad T440 Hot In Russia/brazil - Solid State Drives - AliExpress
Upgrade storage for Thinkpad T440s with SSD m.2 sata 2242 - YouTube
TUTORIAL: Lenovo T440P M.2 SATA SSD Upgrade - YouTube