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:

*Headlines*
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.

*Personalisation*
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).

*Email length*
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.

Thanks,

Nitesh