The absolute tallest off the ground, I suppose, would be this:
1) clear a very large flatish area to the bedrock with your pick.
2) raise terrain support spikes according to your floorplan
3) plant the tallest tree (pine?) at the top of the spikes.
that gives you 20 units tall of ground, + whatever the trees give, at a guess another 30 units?, + natural supported building max height (yet another 30-40)
for a grand total (?) of 80-90 or so units (45 stone columns) tall.
If that won't do, plains rock centerpiece it is... or up side of mountain..