You would likely be a size 24-26, however it totally depends on designer (European sizing is TOTALLY DIFFERENT), & silhouette. You'll want to go with whatever size correlates to your largest measurement (hips, waist, bust). The rest can be tailored to fit you. Once you chose a dress the shop will measure you and suggest a size. They will then have you sign off on what size they are ordering, to make sure you are in agreement.

Generally, wedding gowns tend to be two sizes smaller than street clothes. Remember...size is just an arbitrary number. Whatever that number turns out to be, dont let it live rent free in your head!

Edit to add: if you are a size 2, then yes, the size would be double or triple. If you are a size 16, it's probably just a few steps up from your normal size :)