Mailbox Size field is not visible while adding a mailbox through HC

This usually happens if sold plan to the end user is disabled. If HC billing is enabled and an option is selected to automatically disable user hosting services due to non payment then sold plan will automatically be disabled by HC. Hence HC will not allow to provision additional mailbox.

In order to resolve this issue, settle the pending invoice of the user from Billing Manager :: Customer Report then HC will automatically enable the user hosting services within 24 hrs. To instantly enable the disabled sold plan, login in HC as reseller and go to Hosting Plan :: Sold Plan >>> select the plan which is being used by the user and press enable button. Now webadmin will be able to add new mailbox.