From the very first chapter of SS/PS, "he was almost twice as tall as a normal man and at least five times as wide.". So I'd say 11-12 ft tall. Weight goes up about the square of the height, so minimum 600 lbs, but five times is clearly more than just doubling his height would add so I'll go out on a limb and say about 1000 pounds.
My original guess was imagining a person 2.5x wider than me at the same height and guessing their weight, then quadrupling it. To be somewhat more quantitative, have some numbers:
I'm basing this off a 150-lb, 5'6" - 6' person. Double the size, quadruple the weight for 600 lbs. A torso is 40% of body mass, so making it 2.5x larger after that makes 600*1.6 = 960 pounds. Round to 1000. Bingo! Now, check BMI: 1000 lbs at 12 ft is 34, at 11ft is 40. I was guessing his BMI around 40 anyway, so I'll go with 11', 1000lb. (A bit under 3.5m and 500kg for the non-Americans).