When choosing a length, the most important thing is to focus on your height or the growth of your other half, if it is larger. The minimum height margin should be at least 10 centimeters, but for maximum comfort, preferably more.
As for the width of the bed, it is enough to lie down on it and, throwing your hands behind your head, spread your elbows. If you have enough space in your half, then the bed is right for you. If not, consider other options.
Regarding the shape, we can say that the most rational choice will be in favor of a standard rectangle. But if you pursue an exquisite originality and want a round, oval, arched or even heart-shaped bed, then you are doomed to encounter indispensable difficulties. Not only will it take up more space, but also bed linen will have to be sewn to order. Do not forget about the selection of blankets and rugs.