Break up a file to attach to an email

"You can use 7-zip, it may look a bit confusing but this drop down box can actually be edited with a custom value as well:

(Depending on the email application's implementation, 50 megabytes could be 50000000 bytes, 51200000 bytes or even 52428800 bytes. I'd use 50000000 to be safe as it's the lowest possibility.)"

Just to provide another answer, (the one's above are correct and good) WinRAR will also do it. You can get it from http://www.rarlab.com