Very good information sharing here...very interesting.
I like to robustness and creativity shown.
Most of the workbenches I've worked with have been 24" to 30" deep and the width has varied immensely...but 4 ft to 6 ft width is good...it also depends if your goal is to have a mobile workbench...
We've been using the 4 inch Easy Glide casters from McMaster-Carr.
http://www.mcmaster.com/#standard-casters/=s8d4bw
There are many options at McMaster based on your main goals...
Did you ever post photos of your work bench?