Most weblamil clients only let you send attachments up to 10MB in size. Unfortunately this restriction is becoming more and more out-dated as even simple digital pics can hit 20MB with certain cameras.
One way to get around this is to use HJsplit to break your files down into smaller chunks, which can be recombined at the other end.
Another alternative is signup for a free online hosting service. gHacks maintains a list of sites that offer free hosting. It is worth signing up for one of these services, as they can also be used to backup important files. If you go for this option be careful who you share your login details with though.