Does it have to be retro?
Does it have to be a top end build?
Orange in XS - P7, O2 evo - the 14” frame (12” c bb to c tt) give you a very low standover height, and are brilliant things. I sold one recently here for 120gb frame, F7 forks, and headset.
And the XS O2 evo went round the corner to a friend’s daughter a couple of weeks ago, as a complete build.
Oh...and the XS C16R in slime green went as a full build to a different household in the village.
They come up reasonably regularly - great things. Later ones have disc mounts. Very light. They make excellent road bikes as well as mtbs.
Modern things with very low standover include the brilliant Ragley Marley. I built a medium for the Grom - 27.5 wheeled thing - HUGE bike. Very low standover. It looks like the top tube is down near his ankles when he is riding. I got the frame (2019 model) for 300 and built it from there - very strong, very light. Lovely thing.