Your default mailbox store is located in c:\program files\exchsvr\MDBDATA. The store is the database that holds all your email boxes. You will also find a bunch of .log files as well. These files are used to track changes to the exchange mailbox store. By default, the mailbox store is named priv1.edb and your public folder database is located in pub1.edb. DO NOT DELETE THE LOG FILES!!
If you need to find out the specific size of a user’s mailbox, open up exchange system manager on the exchange server and go to First Administrative Group–>Servers–>yourexchangeservername–></i>–>First Storage Group–>Mailbox Store–>Mailboxes. The users will be listed and the Exchange System Manager will show you the size of each user’s email box.
I hope that helps!