A few rules apply to all systems, then we will get down to specifics. All e-mail systems have limits on size of attachments. Web based e-mail systems limit the total amount of mail they will store. Text e-mail is small. It is the attachments that take space.
Bigger files take much longer to send and receive.
Be sure to use Windows Explorer or My Computer to identify file size before attaching to e-mail. Some allow for more information to be added, like phone numbers, postal addresses, anniversaries, etc. AOL and Juno are more limited than Outlook, Netscape, Eudora and Yahoo. Address and compose the e-mail. Be sure to include a subject.