Um...You're going to have to define "pumpkinrot" a little more. Pumpkinrot is an artist that has made lots of things, but I'm going to go out on a limb and say you are talking about his sentinels.
My work in progress, more pumpkinrot inspired, not trying to copy exactly.
He's 7ft tall, and that head is literally mache over a basketball.
The question is not really HEIGHT to head size, though, it's BODY to head size. And that, you want your head as wide as the chest generally. So, I'd say cut you a couple cardboard body templates (case of soda/beer) at the listed widths and put them up on a stick and get the size/height combo you like best. Doesn't really matter how long of a pole you use.

My work in progress, more pumpkinrot inspired, not trying to copy exactly.
He's 7ft tall, and that head is literally mache over a basketball.

The question is not really HEIGHT to head size, though, it's BODY to head size. And that, you want your head as wide as the chest generally. So, I'd say cut you a couple cardboard body templates (case of soda/beer) at the listed widths and put them up on a stick and get the size/height combo you like best. Doesn't really matter how long of a pole you use.