The answer is: it depends.
Noone can really give you a 100% accuracy answer.
Marketing is all about experimentation, people surveyed different checkout pages and every industry seems to find different results - mainly because their audiences differ. (Age groups, etc)
With email, it's much of the same. That said, here are some tips:
By far your most important piece to email marketing.
Do a small A/B test for the headline to maybe 10% of your mailing list. Your headline is by far the most important part to email marketing - weak headline, low open rate.
Emails coming from a "person" as oppose to a company do really well. And if you can, always address it to the individual (first name etc).
This really depends. You see long sales letters do well, you see short and snappy emails do well.
It depends on the action/service/price you're asking the user for. If it's a free app, as commitment is minimal you can probably keep it short.
If it's a paid app, you may need a follow up sequence and a longer email (or video).
If you provide me with a little more detail, I'm sure I can help - if you even just drop me a message with a little more info about:
1. Your product
2. Your email subscribers (Where you obtained them)
3. Your price point
4. The problem you are solving
5. Any demographic data on your users
I can help you with some more detailed recommendations.
I hope that helps.