The rule of thumb is boots should be a size bigger than your shoes. I use that when I'm wearing waders because the booties are usually larger and I'm wearing thick to very thick socks.

When I'm wet wading with a pair of Simms booties/gravel gaurds & a thin liner sock, I go with my regular shoe size. The Simms is 3mm, but fits snuggly.

If you can, go and try different waders and boots. I like Korkers boots because of the ability to change soles.