I've been using Yahoo since jesus was born and had no idea what my stoage space was limited to or what!! So I just checked:

Users who follow the Yahoo! Terms of Service and who don't trigger our anti-abuse controls can use an unlimited amount of free email storage. You never need to delete another email, unless you want to.

Note: To avoid abuse of our unlimited storage, we do monitor accounts for excessive usage. Yahoo! limits the number and size of messages allowed from a mailbox within a given timeframe. While our goal is to ensure that everyone benefits from unlimited storage, Yahoo! Mail is not designed as a replacement for a mass email campaign solution.

If your account has had a lot of activity in a short period and you are experiencing email response issues, you may need to create new folders or move some emails to other folders.