Definitely sounds like it could be size related. It sounds like everything is working correctly on Keynote's side & the problem is on the email side. A couple of things I can think of:
1. Try to export it using dropbox or skydrive or something like that to make sure that the file can be exported.
2. Try emailing it using a different account (get a free gmail or yahoo or live.com account if you don't have a secondary one to use) to see if that resolves the issue.
3. Try sending it as a Keynote file. A PDF would be way smaller than a Powerpoint file, but I'd think that a Keynote file would be similar in size to a PPT, so you should see similar behavior.
4. If you have another way to view your email account, try exporting the PPT via email, but then save the email, and don't send it. Then go to your email account via the web portal or on a mail app or outlook or whatever on a computer & see if the message shows up with the PPT attached. If it does, Id' defintiely suspect some kind of issue with the size of the PPT attachment.