I started by switching my 150w modeling lights and moved up to 250w. Then I stopped using them for only studying the key light and actually have been able to get a better idea by getting to know my viewing screen on my camera very well to dial in the lighting. I later switched down to a 150w bulb to facilitate fitting inside of the satin glass dome from Elinchrom that I use when mounted on a Mola Beauty Dish. I basically use the modeling light as I said for adjusthing the key and then to aid the focus in the camera.
One thing that does come in as an influence if the modeling light is small when photographing people is that it affects how large the pupil is in the eye. I have not run into this but I've seen it in others work where the pupil was unnatuarally large and I found it distracting.