Like I would tell my students that are a little apprehensive as yourself, "You're thinking too much!" Having the apprehension (aka fear) is a good thing and is something that you should never truly lose. You will find, however, that a weekend on the little 250 will do a lot more to build your confidence that you may think right now.
If it really, really concerns you that a Bonnie is too much for you then by all means don't do it. Start with a 450 or 500. I really do advise, rather strongly, that you dismiss the idea of getting a 250cc. It's just not enough bike for someone your size out on the public roads.
Completing the class is simply your license to continue learning. I've been riding now for over 40 years and I still learn, and I'm still just a little cautious.
