Does a mother of the bride dress need to be floor length?
No! At least, most of the time, no. Black tie, in its strictest interpretation, demands a long gown, but there are many shorter dresses that can be dressy enough to meet the standard. And if the wedding calls for cocktail attire, then you definitely have latitude on length.
Beyond dress code, consider the length most flattering to your size. Take Beth (pictured above in a black asymmetrical Shoshanna dress). She knows that floor length gowns tend to overwhelm her petite frame, so she opted for ankle length instead, which might seem a slight difference, but the hint of ankle actually elongates her silhouette. Tea length (above the ankle) and midi (more like mid calf) can also be good options.
For further guidance, we love the way Vogue breaks it down.