I agree with bruglesco's answer about keeping the same word count. I'll add that it depends on the situation and on the given roleplay session.
If it's a large group of people, your best bet is indeed emulating the behavior of the group, resorting to longer posts only when you want to emphasize certain aspects of the action or if you feel particularly inspired. Keep an eye out for your response time and make sure nobody is waiting too long for you. Don't get too hasty, thought. In my experience, people are willing to wait a little more to get a better fleshed out answer, rather than having to read a flash-answer that doesn't deal with any topic of the roleplay.
In two people roleplays, this may vary. Everyone's is different, but you can always ask the other person preferences about post lenght.
About what details to include, it's ultimately up to your artistic sense. It's a good norm to avoid being too self-centered, e.g. only describing your own character appearence, emotions or thoughs without mentioning the others at all. But since everyone is different, it's hard to have a general rule about what details are useless, and what kind of language is "too flowery". You'll eventually get better by doing it, follow what feels "right" for the scene and the character.