It all depends on what you want to do with the engine. If built reasonably well without tight spots but equally without sloppy fits then with no load most "steam" engines will happily tick over on a couple of psi or 0.1bar.
At the other end of the scale if you want the engine to do real work then you will need a higher pressure as energy in = energy out so may want to use 50psi or 3.5bar.
Size of compressor or boiler will be related to the speed you want to run the engine at, at tick over it will need very little volume. On the other hand if you want it fast to run a dynamo of a racing pond boat than the volume needed to run at 2000rpm will be a lot more.
Two videos of a similar design engine one showing how little is needed, the other at speed but still only a few psi
I would disagree with what PAT says about agreed safe boilers, there are many designs that pass regulations or have been built in accordance with local codes such as the Austrailian code. (AMBSE) and are therefore considered safe. You would do well to get a copy of this code as you are in Australia.
Thats a really nice engine, i think i will make one similar.