There is no limit other than the total webspace included with your hosting plan.
In other words, if you have a Beginner plan with 50 GB storage space, the total space used for your website, emails and databases must not exceed 50 GB.
You decide how you use the space; you can use it all for one email account or host a website with a database and create ten email accounts.
It is, however, not possible to limit the size of an email account. If you exceed your included storage space, you can either upgrade or remove files.