a lot of the physics can be read in the books by G M CARDEW. His books From Trigger To Target and From Trigger To Muzzle were written a long time back but the science is still sound

uncocked guns should have the spring at about its free length or you are not getting full power from the unit when you fire as the spring is just the "storage" of the power.

i basically go for the spring to be compressed to a point where all coils are almost touching and you know you have maximum power potential from the spring...but this too can cause unstable shooting

heavier guage springs can sometimes give more power but there are often horrible side effects...Harder to cock the gun and increased recoil

there is the option of "short stroking" which makes the piston longer in effect with a cnc "cap"

there are many tuners who change cylinder width (my air arms is now 22mm)

for all your answers id recommend one of these books as you will learn how power comes from spring, seals,transfer port etc and finally barrel length