You're going to need a saddle fitter to make sure everything fits, as there's more to consider than the gullet width.
There's a million ways people will tell you how to measure yourself for a saddle, the closest being to add 2" from a Western to an English-type saddle. However, if you're someone like me who has super long thighs, you'll need to take that into consideration so your knees don't get pushed off the knee rolls/blocks. For example, my seat would fit in a 16.5" but I ride in a 17.5" because of my leg.
The easiest way is to try a few different sizes on and see which fits best. If you're taking lessons on lesson horses, the instructor should be able to let you try a few different saddles for sizing. English/dressage saddles are measured differently in the seat; you measure from the middle of the cantle (back of the seat) to the button on the skirts.