How big of mailbox can I have?

or

What effect does a large mailbox have?

the 3 articles below do a good job of explaining those questions.
http://msexchangeteam.com/archive/2007/01/15/432199.aspx http://go.microsoft.com/fwlink/?linkid=3052&kbid=905803
http://msexchangeteam.com/archive/2005/03/14/395229.aspx

The recommendation is related to the item count more than the size of an mbx.

If a mailbox is over 2GB cache mode should NOT be used in Outlook 2007, there have been some changes however large mbx’s can cause performance problmes on the local machine if the mbx is over 2 GB

Its the item count:
Its recommended you keep no more than 3500 – 5000 items in a single folder. This could be sent items, inbox, or any sub folder.

This is not due to the size but when peforming sorts and searches the real I/O happens.

Lets say you have a fold called dogs with 30,000 items this could be excptable but when you want to search for a dog or sort the items this could cause performance problems.