How do I stop an email message from sending that is too large to send on Mac Mail program?

Once you hit send, the only way to stop it, is to disconnect the internet (turn off WiFi, put in Airplane mode, disconnect ethernet, disconnect Bluetooth). If there is nothing to save you can pull the power plug or hold down the power button for several seconds to completely shut off the machine. Once done you can boot in safe mode with the shift key after making sure your wifi router is off and/or ethernet plug unplugged. Then delete the message from your drafts folder.

Note, if the drafts folder was on the server, it is already processing its way out. It will simply reject it if it is too large as an "undeliverable" message.