Length changes a dress more than colour or print does. The same fabric cut short reads playful and cut long reads formal. Here's how the three lengths behave, and which pieces in the shop do each one well.
Mini
A mini puts the whole emphasis on the top half of the dress, so details at the neckline and shoulder do the work. The Eyelet Poppy Mini Dress and the Sienna Smocked Mini Dress both use texture near the bodice for exactly this reason. Minis are the easiest length to wear in heat and the least forgiving to sit down in — worth knowing before a long dinner.
Midi
The midi is the most flexible of the three. It's long enough to feel dressed and short enough to move in. A structured midi like the Raie Frill Shoulder Midi Dress works for occasions where a mini would feel too casual. Watch where the hem lands: mid-calf is the trickiest point on most people, and a few centimetres either side changes the whole silhouette.
Maxi
A maxi is the most forgiving length and the most dramatic. Because the eye travels the full height of the dress, the fabric's movement matters more than the cut. The Sunset Kisses Ruffle Maxi Dress and the Tropical Romance Maxi Dress both rely on that — the ruffles and the print only really read once the dress is in motion.
Choosing quickly
If you're deciding under pressure: pick the mini for heat and daytime, the midi when you don't know the dress code, and the maxi when you want the outfit to be the statement.