I'm curious too, my father-in-law had a huge one on the V-Max I bought from him....needless to say it's gone to the parts bin, along with the factory exhuast.
I have the small factory windscreen. Tried riding without it and at high speeds you can really notice a difference on the chest. Put it back on pretty quick.
If you just want to cut the wind a tiny bit then the little ones help. If you want decent protection at sustained highway speed you'll need a bigger one.
I've had both, and on my next max I'm fitting a medium one.