Here are 2 prom dresses / evening gowns to fit a standard 11.5 - 12” doll.
The Bardot style uses 35g DK yarn. It is knit in two sections - the underskirt and the top skirt/bodice. These can be caught together at hip level or joined using 3 needles.
The mermaid style dress uses around 20g DK yarn.
They are both knit on 3.75mm straight needles and seamed at the back.