You can find this information in your cpanel. In the Mail section, click on Email Accounts. Scroll down the list to find your mailbox. The answer will be under the Usage / Quota column.
If you need to adjust the maximum size of your mailbox click on the Change Quota link. Please do not use an unlimited quota, as this can result in a mailbox that becomes too large to manage.
Note: These totals are recalculated every four hours, so in some cases, the usage amount make not be accurate.