For HO Scale, I believe #4, #6, & #8 reflect small, medium & large respectively. I don't think the scale would change the referece between # and radii. Hope this helps. Jim
Here's a drawing of the Peco Medium N scale turnout, if it helps. The red lines are the rails, the green is the centerline, and the black, which are dimensioned, can be used when drawing up a plan.
As far as I've been able to determine, the ST-242/243 curved turnouts (HO) are approximately a 22"R/19.5" radius. If anyone knows that the radius is for the ST-240/241 is, I sure would appreciate an answer.