ive bought some 38mm keihins for mine, going onto a 2.0 zetec, its not going to be a revmonster and im only going for about 170ish bhp which should be towards their limit.

going on from what chips said, 40mm carbs normally choke down to 30-34mm, 45's normally go down to 34-40mm so anything around 38mm would be spot on for yours as far as flow is concerned, you could end up overdoing it with 42mm on a std 1.6 engine losing grunt, just like you can 'over carb' engines!