18x9 all round is a good size imo (although 9.5's can easily fit), but it really all depends on what look you are going for. I am running 18x9+18 all round on mine, with 235/40 tyres. The rears sit pretty nice, but the fronts need to come out a little bit, I am going to use 15 or 20mm spacers at the front, whenever i get around to it. The C34 front guards can take pretty low offset wheels compared to the rear, especially when everything is stock.
Remember, you can use different offset at the front and at the rear without a problem, it's only having different width wheels that messes the ATTESSA up. So you could go 18x9+15 rear, and 18x9 0 front if you wanted to. Just depends how aggressive you want to go.
This is how mine looks (still needs more low )