First - Can you travel to a local tack shop and "sit" on a few saddles? Deeper seats can be larger and still fit you, then you need to know if you like a narrow twist (or not), and lastly at your height you MAY want smaller saddle flaps.
Once you know what fits you you can tell the saddle fitter that and see what used saddle they carry (if any).
Do you know what tree size fits your horse (narrow, medium, wide, extra wide, something in-between?) Fitter should be able to help with that also - but brand you are interested is good to know.
Dressage is for Trainers!