It might be a bit large. I am 5' 4" and MUCH heavier than you and ride in an 18.
But it wouldn't be outragesouly large and might be just fine, especially if your height is more in your leg than your torso.
I rode in a 17.5" Stubben Siegfried yesterday and it fit me perfectly. I'm 5'9" and 140 pounds. I'm not familiar with the seat on the Tristan as compared with the Siegfried, but I would guess that it would be just a td too roomy for you. I would think the flap and stirrup bar might be in the wrong place for your femur-length more-so than the seat being too big. I'm the same weight as you, and a slightly bigger seat size would have been a little large for me, but not horribly large. However, at 5'9" and very leggy, the flap on that 17.5" saddle was just right for me. You having shorter legs than me (I assume) would probably be a little off balance or chair-seat a little in an even bigger saddle.