Hi Linda,
I hate to say it but that really depends on a lot of things: what your content is, who your audience is, what you qualify as a conversion, among others.
However, in general, the marketer who can most succinctly persuade her audience to act will win. This is true most of the time, though "succinctly" should really probably be "relevantly and succinctly."
I would also say that, assuming "relevant and succinct" are goals, it's not always the length of the email but the placement of a strong call to action. You can have an epically long email (i.e. some newsletters) but if the copy is convincing and your prospect/client can click/tap the CTA without scrolling on their phone, you've probably done a good job.
Oh, if you're doing it right, you're always testing, which means all of what I just wrote could be irrelevant for your audience 🙂
I'm just realizing how long-winded this is. My apologies.
Best,
Nick