Mine is running 20 inch vossens, I think given the roads (from what I can remember) in LA you could go to 21s without a problem; the deep concave does give a slight illusion of being smaller rims than the less concave from some angles. I don't find the ride firm at all on 20s, but then again I had a lowered MINI which was bone shakingly bouncy.
In terms of offset, from memory I went with 25mm at the front and 45mm at the rear as the rear ideally you would want to be 28mm but that's difficult with Vossen who only have 15,25,45 in the larger size rims. I think you should check you might be ok with 25mm in the rear, in the end I got 15mm spacers and the wheel sits correctly at the front and the rear with that.
20s + lowering is the look, the arches are quite big, lowering would help a bit to fill that gap up, I am waiting on someone to have skyhook compatible springs and then it's on like donkey kong.