Very new to watch repairing as a hobby and trying to find the correct mainspring

[lukedysonmt]

I'm currently working on an old cupillard 233 movement and cousins doesn't seem to have a correct mainspring however they do have one that is a bit smaller on barrel diameter is this okay or do I need to look for something else, super new to all this so it's my first mainspring replacement don't want to muck it up

[JohnR725]

Found some information on your movement but also found the mystery. Step dissection now the mainspring Log and that's where we get a mystery.  Apparently your mainspring came in different wits and of course different Lengths.

https://17jewels.info/movements/c/cupillard/cupillard-233/

http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&0&2uswk&Cupillard_233&