i took a CPR class offered through an ambulance service in my area. it only lasted about 2.5 hours, and nothing was done on the floor. we were taught while sitting in desks, took a written test, and if we passed, had to come to the front and perform CPR on a "dummy" that was lying on a table. weight wouldn't be a problem here - the only problem i encountered size-wise is the fact that i'm short, and getting my arms/shoulders in the proper position on the dummy required me to have to raise up on my tip-toes for the chest compressions. however, the instructor DID offer to move the dummy to the floor for us shorter people. there were a few girls shorter than i am, but all of us just raised up on our toes for the chest compressions and did it with the dummy on the table. no biggy
but like another poster said - CPR class is only the beginning of the physical demands that nursing entails. anyone in the healthcare field would be better off to get at a healthy weight beforehand... it will be so much easier on your body, and the work and long hours will be more bearable. plus, it's better for your heart in general. remember, you must get re-certified every 2 years (*edit --for my program anyway.. it might be every ONE year for some). good luck!