Kapusta

Not sure about that bike’s particular sizing , but 18” could have been a med or large, and someone at your height would typically be a large.

That said, conventional wisdom around sizing has changed a lot since then. First, the frame reach and ETT length) trumps seat tube length as the fit criteria, and those have both gotten a bit longer especially the former.

I was riding 17” mtbs in the 90s with TT lengths just over 22” (with a 120-135mm stem and 580mm bars) If I were to ride similar bikes now I would go with a 19” with wider bars and much shorter stem. In reality though, I would be unable to get a good fit on a 90’s mtb now because in order to get the frame reach I want I would be on a large or XL bike and that would just not work.

As far as your bike goes, don’t worry about exposed seat tube. Just worry about if it feels OK. You can always use a riser bar or riser stem to lift the bars if the saddle to bar drop is too great.