Hmm....not sure TBH, but every 'ring I have played on a game has seemed far to narrow (GT4 was a , IRL it is actually quite wide.
Going by that shot, I'd assume that ring is 5 Fiat 500's wide ( 5 x 1.32m = 6.6m), measuring the ring at that point on g earth is roughly 7-8m, which ain't a million miles away.
Not a 100% scientific measurement, other than going there with a tape and measuring it, its the best you are going to get!!!!!.
You can certainly get more than 3 Alfa's through that corner, I have seen two cars overtaking a coach through there with ease.