A strapless dress must fit perfectly to be able to wear with confidence.
Dresses usually come in even sizes: 2, 4, 6, .....etc. The corresponding bust measurements are 32, 34, 36. But most of us will fall somewhere in between sizes. In shopping for the perfect gown, it needs to look great and feel great. Too small and you may have too much back fat. Too big and there could be some slippage when you lift your arms (or dance.)