All this talk about aircraft size is nonsense.
Takeoff/rotation speed is related to the stall speed or in some case minimum control speed of the aircraft. Big aircraft just so happen to usually have a very high stall speed and hence a very high Vr .
Stall speed will vary with density altitude, flaps setting and loading and hence take-off speed will vary with density altitude, flaps setting and loading as well.
As a rule of thumb:
Vr is usually around 1.2 times the stall speed for the current loading which gives good control and a bit of leeway for getting out of ground effect.
If Vr is too close to stall speed you will not have adequate control and risk being stuck in ground effect. If Vr is too high you risk running out of runway and may even stress the landing gear from overspeed.
BUT as stated above look it up in the POH as it may vary.