Vito says on 20/Aug/11
In the show, the top of Higgy's head is almost always at Magnum's shoulder level... 12 inches or so.... therefore, if Magnum is a legitimate 6'4, Higgy was 5'4, very similar to Orville. TC appeared to be about 2 to 3 inches shorter than Magnum, but also wore lifts, so he's about 6' or so.

Carter says on 22/Jul/11
I agree with the comments below. 5'5' or MAX 5'6. His upper body is far too small for a 5'7 guy.

LG69 says on 28/May/11
I saw a publicity shot of John and Tom Selleck for the Magnum P.I. series, and John appears 10" shorter than Tom. They are both standing straight, so the photo serves as a good comparison.We know Tom was a solid 6'4" back then, so I'd put John right at 5'6".
I would put John at 5'6".