Looks like m.2 2280 worked out just fine for t450s : r/thinkpad
Lenovo ThinkPad T450 14in HD Business Laptop Computer, Intel Dual-Core i5-5300U Up to 2.9GHz, 8GB RAM, 256GB SSD, HDMI, 802.11ac WiFi, Bluetooth, Windows 10 Professional (Reused) - Walmart.com
Upgrading a ThinkPad T450s with an M.2 SSD - James Ridgway
Lenovo ThinkPad T450 14in HD Business Laptop Computer, Intel Dual-Core i5-5300U Up to 2.9GHz, 8GB RAM, 256GB SSD, HDMI, 802.11ac WiFi, Bluetooth, Windows 10 Professional (Reused) - Walmart.com
Lenovo ThinkPad T450 i5-5200U 8GB New Drive 480GB SSD 1600x900 A Class NEW 480GB SSD \ Not included | Laptops \ Manufacturer \ Lenovo \ Lenovo ThinkPad | AMSO