There are some new buddleias that are 3'x3' and some 4'x4' ..... these could probably go into a pot that's about 14" high and the same wide.

If you're talking about the normal buddleias though they'll need a BIG pot, it'll need to be at least 18" high and at least 3' wide - these plants have enormous root systems and like anything else - if the pot's too small it's going to die off !

How about buddlea buzz they are ideal for pots.

I find they take a lot of watering when in pots. I have buddleia Buzz in a pot about 14" wide and 18" high. I'll have to put it in a bigger one next year if I can't find a spot in the ground.

I agree Pennyfarthing, anything in a pot needs twice the water !
(and usually a bigger pot!)