I agree with the weight rage you listed, but NOT the height range.

of my own dogs:

Happy-I, and anyone in their right mind who looks at her would say she is Medium. she is 30lbs, but she is 21" tall.

Misty-I also consider Medium, she is 18" tall and 30lbs

Electra-again, Medium she is 17" tall and 30lbs

Blair-my last medium, he is 15" tall and 22lbs

Ripley-I consider small, he is 13" and 18lbs

Perky-also small, at 12" and 20lbs.