Your derailleurs won't actually shift gears unless the drive train is in motion... e.g., unless you're actually pedaling along. Since it sounds like you have friction shifters, just put the levers back where they used to be, and take off again. Everything will work out fine.

Several mentioned bar-end shifters. I didn't think I'd like these, but after riding with them for a couple of months, I love 'em. You get the same utility of friction shifters with an indexed feel, and you can always reset them to work on friction if something goes wrong out on the road somewhere.

And, there's no such thing as being too big too ride, if you get a bike with the appropraite frame size, saddle height, and reach measurements. So go ride, and with a little time and diligence, the rider will shrink.

One of the strongest riders I know will certainly fool ya... She's a big gal, but when she throws down the hammer, you have to hang on with teeth, toenails, and everything else to keep up with her. So don't be dismayed or fooled by body dimensions. It's the engine inside that really counts.