a 40- or 45-series tyre is, IMO, a good compromise between looks, tyre cost, handling, and ride comfort. 35-series is do-able, but then you risk damaging your wheels, particularly if they are cheaper quality (soft alloy).
in general, tyre sizes need to be:
17" wheel: 215/45 or 225/45
18" wheel: 225/40
19" wheel: 225/35 or 235/35
so an 18" wheel is about as big as i'd suggest if 'big' is what you're after. with 19s, you may need the 235/35 tyre in order to have the 'load index' as high as it needs to be for your car (ie: i presume it needs to be 91, but check the sticker on the car).
i had 18s on my 156, and they fitted without probs. you must ensure the wheel width and offset a suitable, and you'll have no 'rub' issues.
with a 7.5" wheel, the offset needs to be around +35 to +40, and on 18s, i'd suggest 225/40 tyres.
you won't fit 20s without having a tyre that's either too big (in overall diameter or load index will be too low). so your speedo will likely be 'out', and IMO they'll look too big and be overly heavy, and quite likely you'll buckle one within 12 months.
pardon the fact that i had chromies back then (!), but this was my car with 18s: