The configuration you have there, if I've read the intent of your diagram right, cannot fire, as the QEV is incorrectly connected. I'd recommend looking at the
piston valves explanation topic, as QEVs work on the same principle as barrel sealing valves.
The approximately volume of the air tank is
The volume of a cylinder is
pi * r^2 * h, not
pi * d * h. The correct volume is closer to 2850 ml.
1. Is there too much power to release all the air once with that volume :bounce:
Yes and no, depending on why you're asking. A ludicrously large chamber won't usually actually be an actual hazard (beyond being a threat to hearing or if you drop it) or stop the launcher working, but using that much air all at once will have no benefit and will just be a load of wasted effort.
If you're intending this for use as a launcher for use in airsoft scenarios, you really want to use the smallest practical amount of air per shot, as you really don't want to spend four minutes pumping it up every time.
That may mean one smaller chamber or working out how to get several shots out of one pressurisation. Personally, I'd probably recommend the first, as it's mechanically much simpler and means the launcher can be completely unpressurised whenever you load it. (Trying to muzzle load a launcher that's still pressurised for several more shots strikes me as a potentially hazardous loading operation).
b. Should I increase or decrease barrel length?
Increasing it would allow more efficient use of the air in the chamber, but ultimately the length should really be defined by what's going to produce a safe muzzle velocity for use in airsoft games.
2. Should I add a smaller or bigger Qev with another so that I can choose to release more or less air ?
I'm not sure what you mean by this.
3. is the air tank too big?
See above answers. Three litres isn't massive if you want to get several shots out of it, but for one shot it would be excessive.
and I want to build an AT4 looking air cannon to use in airsoft game. My plan was to fire different type of ammo. Such as spud, smoke grenade and full loads of BB (shot gun). My plan was to charge it with bike pump only, didn't plan to use compressed air such as co2. 
r and to also shoot a few times instead of one if the target is too close.
