For infantry I use 1/8" magnets and it works just fine.
For larger models (like a big critter on a 40mm or 50mm base) I use a 1/4" magnet. If it is metal, I use two.
(I prolly have over 400 mini's magnetized....)
Check out K&J magnetics - they are one of the best places to get rare earth magnets in quanity.
https://www.kjmagnetics.com/
best of luck!~