The rule goes for many things in life, mechanical and otherwise, but for now think seatposts.

If it don’t fit don’t force it.

Seatposts come in a dizzying array of sizes, some with only .2mm difference in diameter. But that difference is enough to cause some serious frame damage with the improper post. Slightly too big and you risk jamming the post in there, never to be removed again and possibly bulging the seattube. Slightly too small and you’ll have to chronically overtighten the binder bolt, potentially overstressing the top of the seattube and/or binder itself leading to premature failure.

When in doubt, measure or go to the bike shop for a pro to look at it. Buying the proper sized post is a small investment for long term frame longevity.