If your email services are on cPanel, you can launch your cPanel site admin panel & click the "Email accounts" icon. From there you can change the mailbox size for your email accounts.
If your email services are on Smartmail, log into your administrative email account by clicking on the "Manage/Create email accounts" link in your member zone. Once you have logged into Smartmail, you can click the "Email users" option under domain settings & from there, you can change the mail box size for your email users.
The mail box size limit for your email accounts can be set independently for each email.