You'll most likely ride a 250 for the class so you'll get a feel for it there. At 6 feet tall, the 250 felt tiny to me. It's an okay size for training in a parking lot but at the end of the day my legs were killing me because the bike was so cramped.
The 650 really is quite small. It feels big because you are starting out but once you really get out there you'll find that it's a decent size and may decide that it's too small and light. A woman I ride with is barely over 5 feet tall and she sold her 650 and bought a 950 because the 650 just wasn't big enough.
The best suggestion I can offer is to wait until you've taken the class before making any decisions. Assuming that it's the MSF basic rider course that you are taking, they should get you much more comfortable on bikes and that will help your confidence on the 650.
Last edited by krb; 07-31-2012 at 12:31 PM.