Parking radius & aircraft size

I've been playing around with some Airport AFCAD's, how do the gate radius relate to size of the aircraft? The reason I'm asking is, I had 2 DC-10-30 heavies at medium gates (radius 124.7) when there were three heavy gates available. Are you able to set the radius of the gates so just certain class of commercial aircraft use them? I remember reading a topic on the subject several months but I can't remember where. Appreciate the help..........DougD