Hmm what are you after? Aftermarket or genuine AR?
Most owners on here will want to keep their cars fairly standard so an upgrade will be to larger but genuine Alfa wheels. Generally 17 inch are the biggest available, there are a few with 18s but they are a bit big really (imo). Some say that the 17's are actually at the limit of the standard 147 suspension set up, and prefer 16's
If you don't want genuine AR items, most would say you should stick to an Italian designed wheel (eg Alessio etc) for authenticity. Then again it's all up to you!
If you want the best ride comfort / handling go for 16's , If its for looks 17's,the car really suits this size wheel. I'am very happy with 16" Selespeed type alloys but they are also available in 17" size.
I drive 15" during winter and 17" during summer. With 15" the steering is a little light and vague, you can feel the soft rubber absorbing everything and making a barrier between road and the car. With 17" the car has immediate reactions, it has much more feel in the steering and it forces you pay more attention to the road.
I have never driven 147 on 16", but I guess they would be the best compromise for everyday use. Anyway, if the roads where you usually drive are not in especially bad condition, you will be happy with 17" as well.