Increasing mailbox size

I have my emails se up so I can open them with the Zimbra desktop (as POP) and on my iPhone. There are email that I can't delete, so emptying my folders is not an option.

How can I increase the maiblox size?
What can I do to avoid this problem in the future?

Also, my clients keep telling me that they dont get my emails! This is affecting my business, please help!