Since no one else answered I'd try to shed some light... This applies to Positive Displacement (PD) blowers (Eaton, Lysholm, Kenne Bell, Etc.) This is a gross oversimplification, but it should help get some understanding and get you in the ballpark.
PD Blowers are generally rated in Liters or Cubic Inches. Eaton M90, M112 (90ci, 112ci respectively). Kenne Bell rates their blowers in liters.
Essentially that is the theoretical displacement of air per revolution of the blower with no blowby. If you have a 2.0L blower and a 4.0L motor, the ratio of pulley required to achieve 14.7psi of boost is 4:1, meaning the blower must spin four times as fast as the crankshaft. Essentially the blower is forcing out twice as much air as the engine can naturally flow, resulting in 2 times atmospheric pressure in the intake manifold.
Of course there are tradeoffs to be made, and we don't love in a perfect world.
- Blowers have leakage past the screws, which results in less than 100% Volumetric Efficiency, just as engines do. VE is very low at low blower speeds.
- Heat is generated at high blower speeds. Heat is the enemy with PD blowers.
You need to find the balance... larger blowers will require to be turned at lower RPMs to achieve the same boost as a smaller blower on the same engine. This in turn will reduce heat, but it will also reduce available boost levels at low RPMs. This is why strapping larger blowers on mustangs results in more power at even the same boost level. Less heat is a more dense air charge that will result in more power. However all of these cars loose some response in the off-idle area compared to stock.
Like I said, gross oversimplification... but it should help you in deciding on a size of a blower to use.