An occasion dress needs to work while moving, sitting, being photographed and wearing it for several hours. One front-facing catalogue image is not enough.
Check the structural points
For corseted, cupped, boned or rigid-zip styles, bust and ribcage may matter more than the nominal size. For fitted pieces, also consider seat, stomach and rise.
When fabric has no stretch, choose from the largest relevant measurement and consider alterations.
Read fabric and lining details
Georgette, satin, sequins, tulle and lace behave differently. Check stretch, lining and whether seams can be altered.
A sheer fabric or short lining can change how the dress works in real life.
Allow enough time
Order with room for a fitting, exchange and tailoring. For an important event, the predicted delivery date should not fall in the final available week.
Choose shoes and underwear after fitting
Heel height, bra and shapewear can affect length and fit. Choose the dress first, then test it with the pieces you will actually wear.
The goal is not to look thinner. It is to wear a dress that stays comfortable, secure and right for the event.