The composeEmail function on the ScriptedController uses a mailto tag to call your email client.
Limitations on body size vary from browser to browser, and from E-Mail client to E-Mail client.
So we are at the mercy of the browser you are using (each one may have a different length limit for tags and attributes) and also the email client you are using. For more information on what the limits are for particular browsers/email clients try a search like https://www.google.ca/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=mailto+body+limit