I re-tried with your specific dimension and worked fine. I tried entering value via shape data, via Size & Position window. Both as 85.5' and 85' 6". What was less accurate was dragging and stretching the wall. But that's subject to page scale, grid, ruler settings, and zoom level...none of which you provided. Also, need to know which Visio version. All of this so we're on the same page, no pun intended.
The more you can provide, the better someone can answer your question. Also, you didn't upload a file so that we can see what's going on...perhaps something specific to your setup.
Wapperdude