I suspect that a really excellent long post will gain more karma than a comparably excellent shorter post. (But excellence is harder to achieve for long posts.)

I should at this point make the usual remarks that to a good first approximation you shouldn't care about your karma unless it starts changing in major and unexpected ways, but I see you've dealt with this by remarking that you're new here, so karma-watching probably gives you more information than most and karma hazards have more impact on you than most. Even so, if you're contemplating writing something substantial and posting it in Main, the question "what will this do to my karma?" is probably just a substantially inferior proxy for "will this be useful, and be seen as useful, by LW readers?".