I have a golden lab of 4 1/2 months old. He seems quite small to me he weighs 20lbs and is about 17" tall. Is this about the right size for his age?

WEight and height are insufficeient to determine correct weight as the dogs build is all important.

If you can feel ribs and backbone clearly, but not see them sticking out, then they are fine. There should be a slight waist behing the last rib, and the body should feel nicely firm, and not too bony.