This article will show you how to change your mailbox size in Windows Plesk 

Step 1. Log into Plesk. 

Step 2. Click on Mail.


Step 3.  Click on Mail Accounts.

Step 4. Click on the Email account you would like to change the Quota.

Step 5. You can either choose to click on Unlimited or you can click on Another size and type in the amount of size you would like to allocate to the account.

Step 6. Click on Apply.