One of the most striking features of the Tower is its polygonal
columns (fig. 53). Most of the columns are 5 sided, but some are 4 and
6 sided. The larger columns measure 6 to 8 feet in diameter at their
base and taper gradually upward to about 4 feet at the top.
8 feet in diameter is bigger than a half meter; big enough to pitch a tent. But not big enough for some swanky mansion. You could however have several columns in aggregate that presented a 40 foot diameter surface. Or one could have multiple Devil's Towerlets.
Or you could just assert that they did get as big as you want them to be, because I gather your story is not a geological treatise. Something that is big can always be made bigger in fiction. That is one of the chief uses of fiction.