"Choirs can be different in size." is grammatically correct, but sounds a bit unnatural. "Choirs come in many different sizes." or "Choirs can differ in size." are a couple of alternatives.

"Choirs can be different sizes" is correct too.

Or also: Choirs may differ in size.

That is fine. You can also say "Choirs may vary in size"

chorus or choir is usually understood to imply more than one singer per part. Source: https://en.wikipedia.org/wiki/Choir