Most email servers have a limit to how large an attachment they’ll accept. Most limit an attachment size to 5MB (some even less), and if you email somebody a 6MB file, it’s probably going to get “kicked back” to you as undeliverable. Want to get around that? Use iChat instead. Once you have an iChat session started with someone, you can go under the Buddies menu and choose Send File. Navigate your way to the file you want to send, click OK, and the file will be sent to the person you’re chatting with (and a link to download your file will appear in their iChat window). No matter how big the file size is, it’ll get there.