The traditional guidance has been you should ride a frame so that you have only a fistful of seat post showing. On the other hand, that's built around traditional components and having much more than a fistful of post showing meant your post was dangerously extended. That's not so true these days. This allows compact geometry which allows a smaller, lighter, and stiffer frame.

How long your reach needs to be (and thus how long of a stem you need) is a personal thing. A decent shop can help you find your fit, but it's usually around $40 to do so. A good starting place is with a top tube that's the same length as your seat tube. Now stick on maybe a 90mm stem and go from there.