The biggest problem with the OP's issue is that they're 6'3", over 350 lbs., probably about 60 y.o., and out of shape. Conventional wisdom may not be applicable. Be that as it may, get on and off the bike any way you can. Lean the bike way over, that works usually. If that doesn't work, lay the bike down flat on the ground, step one foot over it, reach down and stand it up. When I'm totally exhausted and crampy, laying the bike down flat is the only way I can stagger off it. Nordic skiers can just fall down flat in the snow. Cyclists don't have such an easy option.

Bike fit - read this and apply: How can I fitting my bike

Bike frame fit basically isn't a thing. There's stack and reach, but those are more advanced concepts than called for here. Basically a frame is the right size when it's possible to apply all my fit instructions and not need some weird thing like a 130mm stem or a 50mm setback seatpost to make it work. Go to a large bike shop and get them to mess around with you, looking for the right frame size with the above objectives in mind..

When you stop, you don't put both feet on the ground. and you don't stay in the saddle. You come off the saddle, put one foot down and leave the other on the pedal. I like to put the right foot down in the US, the left foot being more likely to get run over. If you're using platform pedals, you flick the pedal up to the proper height with your lazy foot, push off with the down foot and start pedaling.