It also depends on what category of streams you are trying to model. For instance, do you just want to model perennial streams, or do you want intermittent streams also. The basin size for intermittent streams is obviously small than perennial.
I have recently created modeled streams for our state forests, and I did this by a combination of asking staff to supply points where known streams went from being perennial to being seasonal, and then using those points to calculate a basin size. I then used that basin size in my hydrology analysis as a starting point. I also created several stream layers with smaller and larger basin sizes, and sent all these back to staff asking them to pick the most suitable one.
This basin size does vary by such things as topography, geology, and rainfall, so It's not valid across huge areas. In my case I did it by districts, knowing that it was an average and approximation. You might be able to find information about basin sizes in your area.
