If you wear a size 15 in ordinary shoes, you're probably looking at around size 17 in Nike's, their sizes run small. I figure about 2 sizes off, but I haven't bought them for ages. You might not be able to find those on the shelf anywhere. I wore a size 14 Nike when I was in 6th grade and my feet were still growing, I wear a size 13 now in just about anything else I buy.
If I was buying a Nike, I'd want to try it on first to make sure it fit, that's why I don't like buying shoes online.