Not sure if this is what you are asking...
Contacts came at 50mm diameter or thereabouts when they were first released. Since then Contact have reduced the diameter of the tyres out of the box to around 48mm rear and 46mm fronts. If you mean that these are the different diameters you have seen, then check what's in the box before you buy.
I you mean the question James has answered then... James has answered it!
I run my tyres starting at 44mm front and 46mm rears. 50 fronts wear like iron - in nine races at the NEC for the Autosport my 50 fronts went from 44 mm to 43.6mm! 50s are the front of choice for the majority of SS runners at Nationals. HTH
