What is sufficiently small, and at what altitude? Gases expand as atmospheric pressure lowers as the altitude increases. So whatever diameter your balloon is on the ground is the smallest it will be. The balloon I launched was close to 8’ on the ground when launched. It was closer to 40’ at 90000’ when it burst

They don't care about the balloon, merely the payload. Anything that would matter if something flew into it.

Which is directly related to the size of the balloon.