..

Take it out of the seatpost and the size is usually stamped on the end.

so it doesn;t fit...
likely that iff'n you got the non-fit post inserted at all, then its prolly too small... not too big
iff'n you were able to close the binderbolt/seat clamp and it held the post reasonably, then it was likely not way too small...
so likely it was maybe the next size smaller...
so pull it out, check the size on the post (if no size visible, mic the post) and then go to the LBS and see if they have a 'used' post one size larger you can try to insert. expect the bigger post to be really, really hard to insert at first, since you've already clamped the seattube down some to fit the smaller post...
But iff'n its right, it'll eventually go down properly... if properly greased.
Then do the right thing and buy a new proper sized seatpost from the LBS who 'loaned' you the used test post...
when you leave the bike 'exposed', run a cable thru one seat rail and hook to U-lock or whatever your main lock setup is. A little more hardware to carry, but beats havin to replace the post and seat yet again.

Really, no. I wouldn't. You should use the correct size that fits you seat tube.

The only way to properly use an undersized seat post is to add a shim.

I've used an old inner tube before with success. YMMV

Success is subjective.

That is not the correct way to do it.

If you like it, then go ahead. Good luck.

. . . and your "advice" is a little late. Like to think the OP solved this maybe two years ago at least.

And if he 'solved' it by using the innertube method, by now the seat tube would have deformed, ovalized, or failed. That is, if the bike had been ridden any appreciable amount.

Aw hell, after 2 years now that bike probably has been ham-fisted into oblivion.

Maybe not the correct way but it worked without damage. We're not talking a big difference in size here.

^I had much higher hopes for the post of a 7-year veteran of the site with only 34 posts in total. What a let down.