To access the email account administration area, click on the Email Accounts icon.
1) Landed in the Email Accounts section of cPanel, with the “Email Accounts” tab active.
2) Click the “Disk and Quota Link” next to your chosen email account to reveal the settings.
3) Click the change Quota button to the right of an email address for which you would like to change the mailbox quota.
P/S: The default mailbox quota is 250MB or you can set mailbox quota as Unlimited by checking the option "Unlimited". 
4) Click the "Save" button to confirm the changes. 