Graph it in Photoshop
I, too, am working on designing my darkroom. I posted recently in the "Darkroom Portraits" thread to show my (there was a url link here which no longer exists), also some pictures of the space.
The design was done in Photoshop, with Grid to Gridlines every 1 inch, Subdivisions 2. Measure it lay it out to the closest 1/2 foot. The tables, sink, etc. should be on their own layers so you can move them around to see what fits best.
Having designed several kitchens, I can say that less than 3' for your walkways will be tight, but okay. Two feet will seem impassable.