Solid color dresses in dark hues slenderize and help you look taller if you're short. Wearing your dress around knee length - just above or below the knee or at the knee - looks best. Wearing a mini-length dress can make you look wider because the hem of the dress falls across a thick part of the thigh. Wearing longer length dresses can not only look dowdy, but also does nothing to slenderize.
If you are looking for a print dress, select prints in colors that look good on you and look for design motifs that are proportionate to your size.
Avoid halter dresses unless you wear a jacket with the dress because the halter will tend to make narrow shoulders look even narrower. Also avoid shapeless dress styles, such as shift dresses, trapeze dresses, and tent dresses. None of those styles does anything to flatter your triangle shaped figure.
In the example pictured, the beaded georgette dress (Buy Direct) from Catherines features a V-neckline, a skirt that grazes and then falls away from the hips, a dark color, flutter cap sleeves, a defined waistline, and a V-shaped ruffle on top, all features that would flatter a short woman with a triangle shaped figure