Recently I wrote about emceeing improv shows which gave some good guidelines for emceeing any show. But the venue, audience size, and even audience temperament can change things quite a bit.
The one main rule I use is simple.
Stick with the truth of the show.
Acknowledge if the audience is small, and make that fun and happy. "You will be getting a custom show" etc. Don't wear them out or present as if the theater is full. Maybe get their names so you can talk to them 1:1 if there are only a few.
The truth in a large audience may be that they will be better off coming down together up front if they are spread out. That you need to talk to 10's of people in a different way to get the same thing across.
VERY LARGE AUDIENCE
The truth of a very large or unruly group is that you have more crowd management (or discipline in the case of student groups, etc.) Don't let them run the show, but add some filler as you talk so by the time they are listening you are just getting the the meat of things - I call this a verbal step-down and use it in corporate work and larger classes.
In a Theatersports show you might start with something like "Ok that closed out the round and we are ready for another challenge here as the teams come out" if they are rowdy. By the time you get something like that out (which really says nothing of consequence yet) they will, in most cases, be moving to pay attention and be quiet. Then you can go forward with "Team X, it's your turn to challenge Team Y for the next scene" which is the real announcement.