[EricH]

Useful tip #1: Probably only about half of the new students will wind up continuing on. Plan on about 30 all told (old and new) by the time February rolls around. (Though, according to Murphy's Law, that means that they'll all grab friends and stick around...)

Now, as regards some other issues you've brought up:

Room size can be dealt with. You could have one meeting in two rooms, connected by your choice of methods. You could find a larger meeting space (say, a cafeteria or multipurpose room) for whole-team meetings, and have breakout sessions in classrooms. If you don't have to meet at school, try getting a meeting room at a community center.

How to keep students excited and interested is another fun problem. Vex kits can work... but why not improve last year's robot? How about finding an offseason you'd like to compete in? You could also look up old games, simplify them a bit, and challenge students, working in groups, to come up with a strategy and a robot for that game. (This will also help you in January, right after Kickoff.)

Be prepared for some of the students just standing around doing nothing. There are several ways to deal with this.
Method #1: Homework area. Set aside an area for working on homework if there isn't anything to do. Beats having teachers mad at you when homework is late.
Method #2: Find more work and volunteer someone. CAD, Chairman's, animation, PR, the list goes on. Cleaning the robot work area is also an option.
Method #3: To be used only if a student does not do #1 or #2 and they aren't already working on something. Suggest that they get a good night's sleep at home. "Hey, we don't have anything for you tonight, see you tomorrow!" Rotate who gets that one, though--you don't want to single out one person.