I had a early 90's Giant Rincon that was a size 21" if memory serves me correctly. I think it was considered a mountain/hybrid bike. At 5'9 I thought the bike fit me fine if not just slightly big. I only commuted on it and road riding was just fine. It did not have a sloping top tube like a lot of newer type mountain bikes do.

Your best bet is go out a ride it. When I was younger I felt like I could ride that bike forever. If I took better care of it I probably wouldn't have it just sitting in my moms basement.

Early 90's Mountain bikes can be very different from the later ones. The Rincon had attachement points for two watter bottles and brazeons for both racks and fenders both front and rear. My understanding is that some of them were simply touring frames with 26" wheel sizes. I'd say the road bike frame size would apply.