When designing their wading boots Simms has already taken into account that you will have a 4mm neoprene bootie on. The boots are slightly longer and wider to accommodate this bootie. When ordering Simms wading boots, we have found it is best to go with your straight street shoe size. In other words, if you wear an 11 shoe, then order an 11 boot.
On a related note, wading boots don't come in half sizes, so if you are a half size then go up to the next size. If you wear an 11 1/2 then get a 12.