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

You are completely free to do 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 Mb
SQL 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.