How do we define a small vs. large release? Are there any criteria, such as the size of the container/process volume or vent size?

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.