The dent varries with both object size and speed .
i.e. for the same size bird the frontal area of the dent decreases with increasing speed while the depth of the dent increases with speed. Some similar complicating effects with engine fan blades. It's the combination of dent depth and area for a given speed that relates back to the bird size
For nose domes/cones the hole size is a little more complicated since it depends at what point in the impact and dent sequence the metal begins to tear. Of course if its a flexible non-metalic like fiberglas etc. there may be no visible hole, just a crack which opens during the denting and lets the whole bird inside before springing back closed again.
Simlar effects with ice/hail strikes on aircraft. knowing one or two parameters you can usually work out the others.
With a hard object like a bullet or missle fragment a similar relationship can be worked out at a macro level (TW800 example)
Saw the damage to the front end of an A4k after it collided with a duck at 250kts, the size of the entry 'hole' was rather similar, so perhaps a 1 - 1.5kg feathered missile? Gave the poor old pilot a bit of a start too.