Question

what is the appropriate length to an email

Answer 1

Depends on the cause of the email. If it's a formal email (e.g. at work) a proper length would be between one and three breaks (one if it's just a mention, up to three if you have to explain something (e.g.) - if necessary you can also use more than three breaks).
Every break should have between two and eight lines (maybe more lines, if necessary). And don't forget to format your email - after the salutation a blank line and then starts the first break. And every break should look like a rectangle (and every break should have the same approximate length!). Between two break should be another blank line.
After the last break a new blank line and then something like

Yours sincerly,
<Name>
--
<Signature>


Everything else, like emails to friends or close co-workers can have every length you want to.