Don't worry about your weight, I'm above that and have never had an issue. With the bad knee, you'll want to see if you are alright always pedaling, or if you need to get a bike with a throttle for times when the knee might still get sore. There should be several shops near you, best suggestion I can give is to go try as many as you can out and see what works!
May I ask what your ride of choice is?
I'm 6'6" and clock in at about 285. Spokes and I don't get along very well. If it was just a weight issue I'd get a cargo bike with a 350 pound capacity, but with my height I look like a circus bear on a mini bicycle when I ride those.
I second the Pedego but would choose the Boomerang with mag wheels. If your arthritis acts up you'll appreciate the low step through for mounting. It's featured on their web site as a testimonial from a former 300 lb woman.For someone your height, a lot of bikes would work with minor modifications. At 300 lbs, you're pushing the weight limit of many, but adjusting your spokes regularly will help.
At our Pedego store, we've put people as tall as 6'6" on the Pedego City Commuter Classic 28", but with a longer aftermarket seat post. I think we have someone 6'3" on the stock post.
I'm sure we've put people 6'3" or so on our Pedego Interceptor Classic, but again, you might want a longer seatpost. If necessary, higher handlebars are a pretty easy swap. It's available with a mag wheel upgrade that will handle 400 lbs.
You might consider a day6, they look odd and aren't cheap. They've taken the crank forward concept and moved even more. They even have a model that's rated for 450 pounds. If you're handy with tools you could put your own mid drive (bbs02 or bbshd) in it, or just buy their electric model. http://day6bikes.com.