I really can't speak highly enough of Step Into Life. Its changed my life and probably given me 20 years of living.
Fitball : I lurvvve fitball. I do a SITL fitball class every Friday morning and its great. When I started I used one of the trainers balls for a few weeks and then got my own. Just let the trainer know you don't have one.
Class size varies. My fitball had 3 of us on Friday morning, but then our cardio had about 15 on the saturday morning. It varies with classes and different SITL's.
You mentioned being out in the weather. I still have a copy of the email I first sent to my trainer (before I had been to any class) stating the same thing. In reality it really doesn't rain that much in Melbourne anymore, if it starts raining during class its actually a welcome relief (oooh refreshing) and during all sessions you will sweat and feel exhausted (its exercise afterall) so you tend not to notice the heat (humidity you do though). The trainers are excellent though with the heat. On a 38deg day last week I we worked out in the shade where we had a slight breeze and the trainer brought a big bag of ice for our water bottles, what with extended rest breaks it felt no different.
Bottom line though is the people that SITL attracts. Not sure how they do it but everyone is so unpretentious, supportive and friendly. Theres no lycra bunnies, its the tracky dak and grubby white t-shirt brigade - whether they are superfit or beginners. Everyone's needs and fitness levels are taken into account and catered for but its a team thing and you work as a team. YOu pant together, you whinge together, you take your heart rates together and you leap over kerbs together. Its fun with friends getting fit.
Hmmm boy I can crap on. but hey kinda passionate about how supportive SITL has been for me.