1. Sort by Size, find the biggest email and you will usually discover photos or powerpoint presentations. If you dont need them, ditch them. If you really want to keep them, save them in Dropbox, on a jumpdrive or even on your hard drive.
Messages sent via email do not need to stay in email.
2. Sort by Date. List your emails from oldest to newest and you will discover that you really do have emails from 2007. Delete the relics. Most people can get by with stuff no older than 6 months.
Many email programs allow to to see how many total (not just new) emails you have in your Inbox and other email folders. In MS Office you can right click on an email folder > Choose Properties> Check the box for Show Total Number of Items.