I have got a crossbow with a draw weight of 150 lbs. I want to hunt small game (rabbits, pigeons) at a range of 20 meters (65 feet) and I am looking for a scope.

What dimensions should I take?

Check out the Hawke MAP, great little scope for the money. 3.5 power with several aim point for trajectory.

How much money do you have to spend? For 20 yards the MAP would be my choice too. If you want more potential, the Hawke XB30 or Parker PinPoint are top of the line crossbow scopes.