bobbertyres

Hiya, I've recently bought as XS2 and am looking to get some new tyres/rims. At the moment they're standard at 18" x 4.00 rear and 19" x 3.50 front. I'm using this bike as a donor so i can turn it into a bobber. Can anyone give me some advice on what size to go for. I've been told 16" rear 19"...