kvco vco
Practically speaking, you want the lowest KVCO you can get and still tune over the desired frequency band.
For instance, it you have a 3 volt power supply, and you need to tune from 2 to 2.3 GHz, you need more than 100 MHz per volt for the vtune range available to cover the entire band.
If you made the KVCO much bigger than that, lets say 1000 MHz per volt, you would not gain anything by it, and the VCO would be much more susceptable to picking up stray noise.