Most messages take up very little space, but a deleted message can never be retrieved.
You should save almost all messages, unless they are very large and of no interest. Your program should automatically save all outgoing email in your Out mailbox. You can save all incoming email in mailboxes set aside for different types of mail. Create a separate mailbox for each of your family and friends, labeled with their last name first so they will be stored in alphabetical order. Once you have processed each incoming email, store it in the mailbox named for the person that sent it.