Early bikes were sized pretty much as road bikes - hardly any stand-over and a short top tube made up for by extra wide motorcycle handlebars. By the late 80's this was largely a thing of the past, but I was sized for my first two mountain bikes with less than 5 inches of seatpost showing.
Longer posts that didn't break and long stems were the first real sizing innovations. The top tubes got longer in relation to seat tubes and broke the road bike sizing mould.
You'd fit on a 1987 19.5 Fat. There'll be a fist full of seat post showing and, hopefully, a fairly short stem. It's not great for offroad: you'll nail your nuts the first time you try and dismount if things get too hairy going downhill.
I'd wait for an 18. There are lots.