Hi Michelle!
Yes certainly anyone can start off cycling no matter if they are obese or not, it's for everyone!
It's good to hear you've been making a start on a stationary bike, (even though it can be boring!
) that'll help your body to establish itself to the different way of moving, the pedalling motion, etc. You mention your knees are pretty bad, has the pedalling on your stationary bike been causing you any problems there?
I also think Lisa's comment is spot on. It'll definitely be great for you to get out and about on a bicycle, for one thing it'll be alot more fun than riding a stationary bike, and if you can go out with friends and relatives especially early on as you get back into riding, will build up your confidence greatly!
Have you sought advice from a local bicycle shop about finding a bike that is right for you? A top priority is that you can find a bike that you feel safe and comfortable on and so first try out your local bike shop as they are the best people to help assist you in finding a bike you'll love, and you'll be able to try them all out. They'll know the capacity their bicycles can take so you'll be in safe hands.
Also there are purpose built bicycles for more capacity such as those demanded by body builders who need a stronger bike, so there are certainly stronger bicycles available. Plus take a look at this website
supersizedcycles.com for more information. Hope this helps, and I do hope you'll get back into cycling soon Michelle, do let us know how you're getting on!