If you use Exchange 2003 (I’m not sure if the same applies to later versions), you may have noticed that you cannot set a user’s mailbox size to anything greater than 2097151KB (2GB).
The good news is that this limit is only GUI imposed (soft limit) and not a hard value fixed on the exchange server software it self (hard limit).
Increasing a user’s mailbox is really easy and safe – however, I don’t recommend that you configure the size to some unreasonable or extreme limit.
As always, a full backup of the mail store is recommended before changing any settings on the Exchange.