Yup its a 19", as I measured it before starting to build it up. 20" would be slightly too large for me.
But yours has at least an extra inch of head tube over mine, and they both have level top tubes. We can't both be right - or we're not measuring the same way.
I've just measured mine again - 19" centre to top. 22.5" top tube. It's definitely a 19".
I don't know about '89, but the early nineties Specialized catalogues in the archive here don't list a plain Stumpjumper in 19" - it jumps straight from 18" to 20".
Mine's a Comp.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.