What is the safe level of energy output for hydrogen combustion at home?
First things first: hydrogen gas and oxygen gas do not spontaneously react but need activation. Activation can be provided by a flame, an electric spark or similar.

That said, here is the important part. The reaction will not turn into a fireball but even small volumes will explode with pretty loud bangs. It’s not the loudest bang known to chemistry instructors but it’s certainly something to be aware of – you don’t want your neighbours ringing the police because they think a shooting has been going on.

If you’re going to try this, I recommend starting with the smallest volumes possible, not bigger than a shot glass. My chemistry teacher in high school used a glass cylinder approximately the size of a small (200 ml) water glass. That gave an even stronger bang but the cylinder itself remained unharmed, probably because there was a large opening on one side.

However, the experiment is unlikely to show you much because the water vapour is obviously gaseous and will dissipate rather than being visible anywhere. It would almost be more instructive to identify the two gases as hydrogen (burns when brought to a flame) and oxygen (ignites glimming wood) by the standard procedures used in high school (at least in mine).

Tl;dr: I suggest you don’t try igniting a 2:1 mixture unless you’ve seen it performed recently and know what you’re doing.