There's no good indication from the text that the tower is especially wide or that a cubit in Chiang's world is any different from a cubit in ours. The tower itself is described as being a straight up-and-down tower that deviates less than a finger-width from the base to the top.
The tower's base resembled the first two platforms of an ordinary
ziggurat. There stood a giant square platform some two hundred cubits
on a side and forty cubits high, with a triple staircase against its
south face. Stacked upon that first platform was another level, a
smaller platform reached only by the central stair.
It was atop the
second platform that the tower itself began. It was sixty cubits on a
side and rose like a square pillar that bore the weight of heaven.
Around it wound a gently inclined ramp, cut into the side, that banded
the tower like the leather strip wrapped around the handle of a whip.
No; upon looking again, Hillalum saw that there were two ramps, and
they were intertwined. The outer edge of each ramp was studded with
pillars, not thick but broad, to provide some shade behind them. In
running his gaze up the tower, he saw alternating bands-ramp, brick,
ramp, brick-until they could no longer be distinguished. And still the
tower rose up and up, farther than the eye could see; Hillalum
blinked, and squinted, and grew dizzy. He stumbled backward a couple
steps and turned away with a shudder.
To put the shape into perspective, it looks like this
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/xkLTp.jpg)
But stretching around 270 miles straight up
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/s2ZsN.png)