School size should not really be that big of a deal. 15,000 is not a bad size school and while you like a bigger school idea can be good or bad. More to do in a bigger town, but still more people to get to know!
Joining a GLO is always a benefit. Just try and make sure you check all of them out. Don't just listen to what others say! When you find the right one for you, you will know. Remember, it is not just a 4-5 year commitment but can be forever. The Friends you make will not be in just one GLO, but many. We as GLOs work together in many ways as you will learn.
Good luck for your future endevor!
The best times of your life!!!!!!