I think you have it backwards, loco. The lower #'d frogs are necessarily shorter turnouts. That's not to say that they aren't made longish, but much of what is beyond the frog on both routes is disposable with rail nippers.
Also, be careful about using the notion of "radius" when dealing with N. American style turnouts. The snap track #4s and many European turnouts have a true radius through and beyond the frog with their continuously curved diverging routes, but not the N. American standard. The curve in the points rails is a small portion of a nice wide turn in everything from true #5's and on up, but then they turn straight. So, we talk about a "substitution radius", and that is often a very large number. For example, an NMRA tuned #6 turnout has a substitution radius well in excess of 40". But you can't slip it into a curve because its diverging route is pole straight. You must ease into a curve after that if it is going to be much less than 22-24", particularly for longish engines and cars.
Number 5 turnouts will suffice for most applications, but #6's, if you can fit them in, are really nice on the vast majority of layouts. It is only when you are running long passenger cars at "limited" speeds that you will want turnouts in the #8 and up range, and even then it will be mostly for visual appeal...heavyweight passenger cars will still run through a quality #6 like poo through a goose.
-Crandell