I just tried playing around with a 5m tall cube, as far as I can tell you can’t go larger than 500mm or 20" in any axis because those are the bounding limits in the sliders for the 3D print exporter, as soon as you export through the 3d print exporter it will resize your model to fit within the bounding box defined there.
I basically imported my 5m cube, then tried exporting with the 3d printe exporter, no matter how much I resized it, it is always defined by the 3d print export bounding box.
I was however able to export the correct scale using the ordinary export to .obj in the tool menu. I also have my transpose line calibrated to a 1mm cube so I was able to accurately measure the 5m by dragging the transpose line over its entire length. If I did any scaling in zbrush this was also reflected in the exported obj.
My suggestion would be to import a reference piece that you know the scale of and/or also calibrate your transpose line so you can make accurate measurements within zbrush. Unless someone else knows a work around you might have to stick to decimating, exporting and obj, and then exporting your final stl from your other modelling software