How long is silverstone? in miles if possible?

May be...

3.194 mi

the full circuit from start to finish, 1lap

3.194 miles

there's different lenghts for different championships

The outer most circuit measures up as the one used in

F1 Grand Prix is 3.1940 Miles

Minor users of the circuit are

International circuit 2.2490 Miles
National circuit 1.6390 Miles
Stowe circuit 0.7960 Miles

All these minor users have different circuits running in between the outer circuit hence all the differnt circuit mileage and pattern