The Cushion shape is the antique-look cousin of the round brilliant. Comes in many dimensions with only few júst right, be it oblong or squarish. In other words, Length/Width Ratio: Whatever you like! Most of them though carry too much weight in their belly, making them appear smaller than what their carat weight says.
Red flag: Choose well between an oblong or more squarish shape and also, make sure you are informed about the difference between the ‘Crushed ice’ shape and the antique, chunkier look.