Most machines power between 15 and 100 psi, that is not an issue as you can adjust the force with the control switch on most guns.
Nozzel sizes are betweem 0.25 ( it is millimetres Dave
) and 0.75
I use a few different guns from large 0.75 ones down to 0.25 but My favourite all rounder is 0.3
which is this gun on ebay
300242826266
It is as good as the £150 guns.I prefere the gun type to th pen type guns and also find better flow from gravity fed guns rather than suction fed as they tend to spit more with lower psi an smaller compresors.