5 gallon... BB's need room. sure you can put a cage on top its just wire and will bend accordingly. I prefer not to use pots for numerous reasons, dry out, cramped ect. but some times you have to do what you have to do......you might try Home D buckets and drill holes in the bottom.

How large the pot is ? Most recommend 10 to 12 gallon size. The bigger the pot, easier to maintain consistent moisture level and less frequent watering.

Better Boy is an indeterminate and can grow upwards of 5 ft. So the cage has to be over 4ft tall (above the top of soil) then you let the foliage drape.

Sey

If you have the pot on the ground, then drive at least one sturdy stake deep into the ground next to it and tie the cage securely to the stake in multiple locations. If you only sink the cage into the pot, the whole thing will be very unstable once the plants gets big or if you have a storm.

Container size depends on the plant but Better Boy is a good size plant. It would do ok in 10 gallons but personally I would not use less. It all depends on how much success you want for the plant. Bigger containers give much better performance when dealing with indeterminates.

Dave

PS: and the soil-less mix you use to fill any container is vital. Container Gardening forum here has lots of good info on that aspect.