Roundover bits are very expensive...if it's not visible (mine is on top and 7' tall) you can carefully cut a hole with a jigsaw at ever-increasing angles to get a wider hole (I used two passes). I think it's not half bad, though it isn't nearly as fine a job as others I've seen. It turned out increasing the port radius by 1" (8" port has 10" flared opening)
Depends on how you join the wood. If you use a miter joint you can go farther. With a butt joing You're fairly limited. Just make sure you get a good bit. Bad bits can kill you. I recommend Whiteside bits.
I try to keep roundover to a minimum. Especially if you use screws or nails.