Ive got my punto mk2 hgt lowered 50mm, jus wondered what size rims i could fit on without any scrubbing, or arch work?

it depends on the offset of the wheels, the width of the wheels, the tyre size you use, and how much arch work you are you willing to do.

i bet 20" is easy enough with rolled arches.

with standard arches i think 18s will be the biggest you can go, and i bet they need to be 5.5" wide with 215/40/18 tyres.

the biggest you can go without looking like a dork depends more on the style of the wheel, but imo 17" is the limit in all cases.