I was wondering if a 2/0 wire will fit in a siemens 100 breaker?

Thanks for your help

You probably have a 100 Amp QP Frame, the largest wire it can handle is a #1/0.
See this spec sheet:
http://www.sea.siemens.com/SpeedFax06/Speedfax06files/06Speedfaxpdfs/06Speedfax_01/01_32.pdf

Just had another question about this issue:
https://www.askmehelpdesk.com/electrical-lighting/00-100a-breaker-41405.html