There is no consensus on the “correct” answer. Small leaks of short duration have a much lower
probability of ignition compared to large releases. Ignition probability is affected by the operating
conditions, whether the release is from a leak or vent stack, and the surrounding environment. Since the
probability of ignition is high, hazard analyses will usually assume the hydrogen will likely ignite and
should consider the potential consequences of ignition to the system and surroundings.

A release is defined by the amount of hydrogen, the rate of hydrogen flow, vent location (indoors or outdoors), geometry in the area (confined or not), and pressure. 

A small or large release should be differentiated by the damage that can occur because of an ignition. This can be a fire, deflagration, or detonation.

Therefore, the relative size of the release will vary based on all the variables above.

If outdoors in an unobstructed area, a small release could be defined as a release that keeps the flammable hydrogen cloud from damaging its surroundings.