Help! model cut but showing full size

I split a model for 3d print and used the knife brush to cut the model from the knee down, zbrush shows the size as the full size before cut as if part was just hidden, but when I select delete hidden or show part it isn’t there, when I measure the part that I can see it is the size I want. Then when taken into prusa slicer it sees it as a whole part, only the top part is not visible but is still recognizing it as full size and taking that much room on the build plate. Does anyone know how to fix this? I hope this makes sense, it was hard to describe…

I figured out how to make it work. I had to load a model in and append the calves and feet in with the other model (not touching) then boolean meshed them together and hid the model I didn’t need and deleted it. The part that I wanted finally just measured the part that was still there rather than the whole model that I cut it from. Probably a better way to do it LOL but at least it worked!