Does anyone have an opinion on how wide I should make the front tyres on a non-power assisted car (it's a Spire GT-R). The car will weigh about 500kg's. I'm using a 2.4 quick rack and i've got a 270mm diameter steering wheel. I don't want my car to tram line and I don't want to have to grow biceps the size of He-Man to be able to park the car. I was thinking 16" is a nice size but not sure on width due to above issues.

Cheers

What do most Spires have on? What do Spire recommend?

16" seems big to me on a light car. Large wheels and low profile tyres on a light car give poor ride and handling in most cases. Thay're also heavier so increase unsprung weight.
13/14/15" are more common on kits. Odd sizes, as opposed to even, tend to have a greater variety of tyres available as well. Go for 13"/15" (17s are too big) for more selection of tyres.

I think either 13 or 15 inch wheels 7 inches wide or so would be about right