In this real life rider comparison we are going to look at Brooke in the same model of saddle, the Voltaire Palm Beach, in 2 different sizes. Brooke is 5' 8" and wears a size 30 breech.
In the photo on the left, we see her in the 17" 2AA flap Voltaire Palm Beach. Although the flap looks like the right shape for her, her knee is right at the edge of the flap and overall the 2 flap looks small in surface area. In the picture on the right, we see Brooke in a 17.5" 3AA. You can see the flap looks roomier and her knee is not at the edge of the flap as it is in the 2AA.
Now lets compare the seat. In the 17" Brooke looks to have about 3 fingers width from the back of her seat to the cantle. In the 17.5" she has closer to the traditional 4 or 5 fingers between the rider and the cantle.
So the answer is, yes, she does need a bigger saddle! While she could probably make the smaller saddle work fine, overall she will be happier in the bigger saddle.