I wouldn't say that size (by which I mean weight, really) is usually much of an issue in dancing.
Much more important is the way that we carry our weight: our posture or carriage, and whether we support our own body mass properly (particularly from the upper body on turning figures), rather than using our partner for balance or support.
I watch a seriously overweight middle-aged man at a regular social dance I attend, and in fast-moving dances like Jive he is amazingly "light" on his feet, and moves with ease and style, while plenty of people who are as thin as rakes, move like planks.
Having said that: being overweight isn't healthy, and we should all take better care of our own bodies. Setting aside a few medical conditions (which are very rare in the general population), being overweight is the inevitable result of eating too much. The solution lies in our own hands, but dance is a good way to obtain gentle exercise - one ingredient in a healthy lifestyle.