It depends...
The track used for G scale has been used with 1/32, 1/29, 1/25, 1/24, 1/22.5 scale models. If we are talking about a 6' man, yes in 1/24 he would be 3" tall. But in 1/29, he would be close to 2.5". In 1/22.5, he would be approaching 3.25".
Of course, adult males can be shorter or taller than 6'.
