First I pick one wall, if I want to change vertical I pick the wall at the horizontal, if its horizontal, well I pick vertical. Let me show you an example.
In this I pick the vertical one then would use the create dimensions. I would zoom in the image, put my cursor on the wall I want to check, in this case the horizontal one and move the vertical wall, with the keyboard arrow till I get my 8'8" size.
They might be an easier way to do this, but for me this work very well.