How to send a email with a zip file between 15-20Mb

web mail usually has limit on mail size
for yahoo, the limit is max. 10M each mail
so, your case is impossible and also not practicle
people will have difficulty and have to wait a long time to finish downloading that mail
i would suggest you upload that file to some free online web storage and give your friend the link to get that file online
you might use yousendit: http://www.yousendit.com

Use the link below to sent the file:
http://www.yousendit.com/
If use 1 time only, no need to sign up.