When setting up a new email account cPanel will automatically default the account to 250MB space.  For most clients that will be suffucuent but if you want to choose Unlimited you can.

It's important to note that Unlimited in this context just means the mail account will not have any limited but you are still limited to the amount of space allocated to your web hosting account (where the emails are stored).  So let's say you are on a Standard Account with 40GB Disk Space and choose 'Unlimited' for your email account then your email is technically limited to 40GB as your emails are stored in your hosting account