What should I enter in the size limit of the mailbox?

You are completely free in this, as long as it is within the space of your package.

So this can be from 1 to 10000 Mb.

Example:
You have the total package:

Website space 300 MbSQL
Database 50 Mb

You then have 9650 Mb left for email storage. Suppose you have 3 email addresses then you can divide the space between these boxes.