Google Mail is awesome. You can use it as an alternative to exchange, or even for personal use. As some GApps administrators may have seen in the past, users love to archive everything (you may even do this in your account). Google gives you 25GB of space for your Mail account, which may seem like a lot, yet there are some users who use all of it.
In order to find those large messages and attachments there is an undocumented size search operator that you can use, which will find mail messages greater than a certain size. Example search queries below, which can be clicked on to search within your Google Mail account:
Find Messages/Attachments Greater than 5MB:
Find Messages/Attachments Greater than 10MB:
Find Messages/Attachments Greater than 20MB:
** Additional Tip ** You also have the option of using: size:5MB, size:10MB, size:20MB, etc as well.