You can use literally any size.
If you use a small one
a. You will need to position it much further away, the light will have to travel much further and so you will need much more flash energy because the light loses power over distance
b. Because it is both small and far away, the light will be much harder - this is not necessarily a bad thing, whether it's good or bad depends on the effect you wan to achieve
c. Because it is far away, there will be much less difference in the distance that the light which goes past your subject has to travel before it hits the background, therefore the background will be much brighter than if it is close to the subject.