Mark,
It was loosely based upon a plan for a "Tools and Lumber Cart" in Fine woodworking. That design had cabinets / workbench on one side with sheet good storage behind. I've included only one page from the PDF because the document is copyrighted however it shows the basic framing technique.
Pages from Tool -n- Lumber Storage Cart.pdf
My structure is 8' long, to accomodate a 4'x8' sheet of plywood. You will note the framing plan shows a "shelf" above the sheet goods rack and below the top to add rigidity to the structure. I sized mine to put the horizontal support for that shelf 62" above the "floor" of the structure to permit me to store 5'x5' pieces of baltic birch. I sized the shelf area to give me about 3" or so of storage space there. That provides me a place to store my 4' level, long straightedge, 4' T-square, circular saw guides and a few long small scraps of wood.
In an effort to balance overall width vs. board storage, I used 10.5" KV 82/182 shelf supports / standards from WWS which give me full-height shelf adjustability in about 1" increments. I used 7 supports per shelf, mounted 16" on center, including both ends. Each shelf is only rated to about 300# however with the load fairly evenly distributed across 7 supports, each shelf is heck-for-stout. I decided to make the overall structure 40" wide, leaving about 12" width for sheet goods after framing.
If I were to do it again, I would treat the casters differently. I installed a total of 10 small (~2") locking / footed casters (zoom in on picture), with 6 under the load (under the shelf standard) plus one under each corner to prevent tipping (we can get earthquakes in Alaska). With those small wheels it doesn't take much to block the rolling and one of the 10 is always finding something, lest I use my compressor to blow the floor clean before moving. I then mounted a long power strip on the front end (~24" and ~12 outlets). I use this for my batttery chargers, the lights on top, and the occasional dremel, sawzall or other convenience tool.
Jim in Alaska