personal experience and common sense are the 2 main sources for numbers estimation. My advice is to go with easy-to-use number (e.g. instead of 47 use 50) and to check 1 or 2 important and uncertain numbers with interviewer (e.g. "I'm not familiar with this market, can I proceed with this number?" or "does it make sense?").
I share some examples of market sizing with some good number estimation arguments:
Hope it helps,