I'm doing Body Combat and I'm usually the only really big person there but I just try my best. There's a couple parts that I modify a little bit, but I am lucky because the instructor is amazing she encourages modifications if you need it and she's overall great. So if you have a great instructor in my opinion that will make a big difference in comfort level. The same class at another location the instructor was really snooty and I never went back to her after that one class. Good luck!

I just started a step class last week at the gym (though have been doing them in my basement for two months prior) and did a weights class for the first time this week. Ere are people of all sizes and fitness levels. You'll do fine. Just start with no riser on the step to test it out and light weights for the weight class to try it out. Next time you can increase if you think you can.