This car is a little older but you could use it as a guidline for appearances. I think a 15 inch wheel would be ok but you should concentrate on a taller tire size instead of a wider tire size. If you were using bias ply tires I would recommend a J78x15. Notice that for a true gasser look you should have a straight front axle.
edit: The style of wheels that car used is pretty typical of those used on gassers in the sixties, as you said, a five spoke pattern. The term "Radir" comes to mind, I think they were a wheel manufacturer back then.
second edit: I just found
this link, I think it may help.