skipp, i wear contacts and have no probs with goggles.
basically, i find if i go to a sports shop, put the eye bits on, but not the strap over your head, and just feel for comfort. When you wear them the strap should be tight enough (but not cut the circulation off to your face) to hold a good seal, but as Jj says, it is to some extent trial and error. I get on well with most speedo styles, and always buy a pair with adjustable noe bit, although other people find the all in one moulded silicone ones best.
Prescription goggles are expensive, and if you're as blind as i am then you a) can't see the changing rooms when you take them off after your session, and b) can't take them off to do kick drills etc.