When hydrogen filled balloon rises high in the atmosphere:

It will expand

it will shrink in size

it's size will remain unchanged

will depend on air pressure

A hydrogen-filled balloon expands as it rises and may even burst after rising very high in the atmosphere

As one goes high, the weight of the atmosphere above decreases and so does the pressure.

Thus for a balloon filled on the ground, the pressure inside the balloon exceeds that outside and thus it bursts.