I would try to find a tack shop or online consignment shop that will let you try a saddle for a week or so. You may need an 18", depending on how tall you are and your rear end size. My daughter is only 5' 6" and my husband's 18" saddle isn't that big on her. Her friend's 17" is much too small for her hiney, lol. She wears a size 8 in pants and should be in a 17.5", but she's not done growing (she's only 11!).
A 17.5" with a flatter seat and a more forward flap might be fine, but you won't know until you try one. An 18" probably wouldn't be "too big" if you want to play it safe