Sending unit terminal nut size????

. But you should not need a nut. The original wire has a "push on" 90* connector
The origional needs the nut to seal the insulator to the sender.

After markets don't need that.see flange in the photo post 3

OEM sender no flange
PXL_20220410_201201610.jpg
Aftermarket sender with the OEM sender post next to it. itScreenshot_20220410-141626.png