Apps for Business includes 25GB of Gmail archive. If you're starting to run out of space, one way to address this issue is to delete some of your largest attachments.
First run a search for your largest attachments, then move them to Google Drive, and then delete email messages with the largest attachments to free up some of your archive. In general, you should try to avoid large attachments and instead use Google Drive. You can always purchase more Google Drive space, but there's currently no way to increase the size of your Gmail archive. Here's an example of a search for messages with attachments over 15 MB: