If you halve the chamber size but double the pressure, you'll get one huge amount more performance.

I had done a little exercise with GGDT

here for an imaginary 0.177" BB launcher to prove this, these were the numbers I got:

20 inch chamber at 50 psi - 406 feet per second

10 inch chamber at 100 psi - 537 feet per second

5 inch chamber at 200 psi - 684 feet per second

2.5 inch chamber at 400 psi - 830 feet per second

1.25 inch chamber at 800 psi - 958 feet per second

0.625 inch chamber at 1600 psi - 1006 feet per second

Remember this is the same quantity of air, distributed differently - and the results are quite compelling, especially considering that for a 100% increase in velocity, you get a 200% increase in muzzle energy, so in the first case, velocity increases by 32% but the energy increase would be in the order of (edit - thanks Ragnarok) 74%.

Regarding the silencer, for this launcher:

Assume one builds a pneumatic launcher with 3/4" barrel, 30" long. The chamber is made of a 1" dia pipe, 8" long and pressurized to 350psi.

The

ideal volume of silencer + barrel would be equal to the volume the air in the chamber would displace if it was at atmospheric pressure.

In this case, chamber volume is 6.28 cubic inches, multiplied by (350/14.7) gives us 149.50 cubic inches. Barrel volume is 13.25 cubic inches, so total silencer volume should be around 136 cubic inches.

Say you're using 3" diameter pipe as a silencer body and having it coaxial with the barrel (assuming the barrel is 1" outer diameter), this would mean it has to be 20 inches long.

Again, this is the ideal volume, which is rarely practical in terms of size and bulk - in practice, I would say make it as big as you can afford.