When I joined my gym 5 years ago, I weighed in at 212 lbs. I'm only 5'3. I went through the pump training at a weight of 195-ish, through the step training at 185, through Attack at 176 and finally through flow at 165.
I had reservations with each training because of my size, but I LOVE the classes and love teaching them even more. If I had waited to "look fit" before training, I'd just now being signing up for Pump...maybe. I'm down to 152 (yay) and in a US size 8 which feels pretty darned good actually. My goal is 140 and I'll take as long as I need getting there. Guess I should add another program, huh?
I often wondered how participants initially perceive me, but they come back, so I must be ok. I've had women bring friends to MY class because they knew I wouldn't be intimidating to someone "larger". Too many overweight people put off going to the gym because they want to wait until they are in better shape, or because they feel judged by others or because of fears they won't be able to do everything in class. They might identify with a larger instructor and feel more at ease and be more likely to come back! Sure, there will be people who dislike or judge an instructor who doesn't have a role model body, but as long as the instructor can demonstrate all the levels with great technique and coach the heck out of the class then so what?
Of course, as an instructor we want to be able to do everything and dropping the extra weight sure does make doing everything a bit easier. Especially in Attack. I can't say that I'm any more intense teaching now than I was at a heavier weight, but I will say I feel better after class.
Don't wait on training. If you love it, then do it. For every participant who wants their instructor to have the six pack there are three who just want someone to rock their world for 55 minutes and give them an amazing workout. You don't need visible abs of steel to do that for them.