Kent -You're confused by all the numbers.
Here's the thing: when it comes to "physical size," ALL Airheads are the same. That's because they all have the same frame and wheels. They are "parts bin" bikes.
When you see 500, 600, 750, 800, 900, or 1000, the only differences are in size of cylinders and cylinder heads, carbs, and rear drive ratios.
Seats are all the same, foot pegs are all the same in the same locations, there are about 5 different handlebar shapes, making the only ergonometric differences. A fairing may make the bike look bigger, but it's the same bike, heavier obviously.
Here are the VERY FEW exceptions:
In mid 1973 the wheelbase was lengthened. Basically you should avoid the earlier versions.
In 1985 the frame/wheels/suspension were significantly modified, but overall dimensions changed very little.
From 1979 to ~1984 there was the R65, which indeed is a physically smaller bike. The R65 of 1985 is not.
From 1981 to 1986, there was the R80G/S, which is different but not particularly smaller.
The R80ST is basically the same as the R80G/S, but smaller front wheel brings it close to R65 size.
The R100GS and R100R are a unique pair and a little longer than the G/S and ST.
That's all there is.
Compared to a modern BMW like for example the R1200RT, all Airheads are small. The F800s are similar in size, but have more power and significantly better fuel economy. The Airhead "soul" is still there in the R1200s, not at all in the F-models.
PS: physical size is often psychological but the physics is real. An R100RS with its itty bitty handlebars is a heavy pig, a G/S or GS with wide handlebars seems a featherweight in comparison. That's how they comparatively handle, too.
Yes...confused for sure.
But you information has been really helpful. THANKS!