The usual question would be "How much do you weigh?"
"How heavy are you?" has a negative connotation - it makes it sound as if you already think the person is "heavy" (ie overweight).
Please note that unless it is for safety reasons to do with work, this is not a question you would ask people in the UK. It's considered rather personal!
If you have a question about the English language and would like to ask one of our many English teachers and language experts, please click the button below to let us know: