Mailbox sizes can be increased by the related Webnames account owner.
Or, to manually manage your domain's email accounts:
Log into your Webnames.ca account
Navigate to My Account > Email Accounts
Under the applicable domain name click the Manage button.
Under the Modify heading, change the Package Type to reflect your desired storage amount.
NOTE: When upgrading (or downgrading), all email accounts associated with a particular domain name must all have the same storage amount.
Click Apply and proceed through the shopping cart to complete your upgrade purchase.