Old style (pre 90's) frame sizes

I was looking around for an old mtb, pre 90's, and I was wondering how frame sizes compare from that time with more modern sizes. I mean, I like 19-20" for a mid 90's bike and 58 for road, but I have seen pre 90's bikes that have frame sizes up till 58cm, which would be 23"! That is pretty big, but the bikes don't look big at all. Any opinions on this? Would a 20" bike be to small for me?
Thanks!

In my experience the older frames have longer seat tubes by design or if you wish shorter top tubes for a given seat tube size. Many frame designers used near horizontal top tubes so that figures.

Around 1990's the cockpit was also fitted with long zero rise stems, and often short head tubes. My 1990 Explosif 20" came with a stock 150mm zero rise quill stem. My 1990 Stumpjumper 19" is a fair bit lower in the headtube again and has a stock 140mm stem. Both frames are non suspension corrected with a 390mm fork axle to crown length.

I'd say a 20" would probably be OK, but experiment with the stem/bar to find your riding position. (I'm 6'2" btw with a long inseam leg and like similar sized bikes as what you mention, maybe 60 for road)

Enjoy!!

Fit depends a lot on what else you ride. When those bikes were designed, most buyers were used to road bikes and were comfortable with low handlebar positions, head down. Also forks were rigid, and the kind of cross-country riding people did was fast, relatively long distances, the kind of thing a rigid fork is most suitable for. And rigid forks suit longer stems, weight more forward, so top tubes were shorter to complement the long stems.

Now people who buy retro bikes tend to already have a long-travel modern hardtail, with a short stem, long top tube, high handlebar and head up style. And they go for shorter rides in the woods exploiting the potential of the forks. So you see them wanting to set up their retro bike in the same kind of shape. Hence you can buy 150mm stems for pennies because nobody uses them, even for bikes that were designed for them, and you see people with rigid forks, short stems and riser bars.

But it sounds as though you might be fine on an early size 20, provided you set it up the way it was designed to be set up.

consider that many modern frame sizes are not real but virtual ie they are treated as if the top tube was horizontal like normal bikes. Also fashions change and once we rode larger frames , just like road frames are shrinking for a given body size. When I bought my first mtb (87??) we sized them 2 or 3 inches smaller than our road frames which were traditionally about 10" less than your inside leg. so mine was 18" with a horizontal top tube for a 30" leg. My bars were lower than the seat, roughly level with the stem clamp, the stem maybe having a bit of a rise or horizontal rather than the normal 7.

Sounds like it could be a bit small. I normally ride an 18-19 inch frame but my 1985 Miyata is a 20 inch and if I'm honest a tad small. It's fun to ride but I wouldn't want to spend the entire day on it.

A lot of the older bikes had short seat posts but also layback ones so reach isn't usually the problem - but height is.