By default, only 25 emails are displayed on a page. You can change this by opeing your email account and clicking on "Options" in the upper right corner (in blue letters.) A page called "Mail Options" will open. Click on "General Preferences" in the center column (also in blue letters). In the section called Folders/Search Results you will see a drop down box showing 25 messages/results per page. You can change that to the maximum of 200 messages/results per page. Click the Save button in the lower left or upper left corner of the page.
However, the only other way to delete more than 200 is if the majority of the e-mail is from a particular source. By using the search bar at the top of your email, it will return all hits of that string. So, if you're getting spammed by Twitter emails, you could search for the string "twitter", and delete them all at once.
Click here for a youtube video of this search method.
Edited by Fusionbomb, 19 March 2015 - 09:48 AM.