Size doesn't matter. Neither does age. I'm happy to report that this look is great on everyone. A long dress can absolutely be worn by an older woman; many styles offer arm coverage if that makes you feel more comfortable. (If the clothes tend toward bare, a jacket thrown on top only adds to the casual appeal.)
Quick Tip: When combining patterns, look for a unifying factor—similar scale or color scheme.
Jacket, Etro, $1,647. Dress, Nada, $500. Sandals, Call It Spring.