smokin, During the last 30 years or so I have made 40 or 50 workbenches from materials purchased at lumber yards. Some are free standing and moveable, some have shelving built in above or below the bench top, some are built onto the wall.
Here is some of what I have learned: 1) How tall are you? Decide how high you want the bench. Will you be standing or sitting while working on this bench. Will you be sharpening chain and other small tasks or will you be assembling "things"? You probably do not want to go much higher than 24" if you will be working on big things like snowblowers, lawnmowers, wheel barrows, assembling furniture, etc. If you will be standing while working on chain saws and similar tools you probably do not want to higher than 37". My kitchen counters and reloading bench are 37" high, yet I stand at one and sit at the other. Most of my work benches are between 27" and 32" high; 2) The length of a built-in work bench can be as long as your space permits. If the bench is free standing and you will be walking around it, do not go longer than 8'. Any longer makes walking around it very annoying; 3) Use wood efficiently; Do not go deeper than 24" (half sheet of plywood) if the shelf is built onto a wall: If the bench is free standing and you will be working on it from both sides, 4' (full sheet of plywood) is as deep as you will want to go. At 4' you can still easily reach to the center of the bench from front and back sides. One of my work benches is 4'x8' with 4x4 legs, electric outlets, a single shelf, and all kinds of shallow hooks used to hang things. My friends call it my "elephant operating table" and would love to own one.
Shelves: A single shelf is about right for under the top of the bench. The top of the bottom shelf should be at least 16" ( I prefer 18") below the top of the bench because you will probably have a 2x4 frame supporting the top and that will take away 3.5" of those 16". Many of the buckets, containers and other things we use to store things are about 12" high. Five gallon buckets range from 13" to 14.5" high. Trying to maneuver a heavy 5 gal. bucket under a 12" opening is a pain in the butt. If you build two shelves below the top of the bench, one of the shelves will be kind of okay and the other will be so shallow, top to bottom, that you will use many naughty words trying to find things at the back of the shelf. A shallow shelf at head level is okay but at knee level it ain't too much fun.
Work surfaces at top of bench and shelf: 2x4's and 2x6's make strong but uneven work surfaces. I prefer a good quality OSB -Advantec is my favorite. It is at least as strong as plywood, significantly flatter, and easier to work with. The top should be two layers of 3/4" OSB glued together and screwed down to the 2x4 frame. This will easily handle any weight you can place on top of the bench. The shelf has only a single layer of OSB. Build a strong frame of 2x4"s with front to back stringers at 24"on center (o.c.). Purchase a couple sheets of 3/16" or 1/4" (1/4" is better and my favorite) Masonite (pegboard without the holes) for the work surfaces. Masonite sheets are not expensive and are a little bigger than 4'x8'. This will be ripped in half and screwed down to the OSB top and shelf. A couple of screws on each end and a couple in the middle is all you need. These screws just hold the Masonite in place. The surface of the Masonite is shiney, slippery, hard and durable. You will have to countersink the screw holes to drive the flat head screws flush with the surface. One of the many benefits of Masonite is that when you ding it up with drill bits, saws, oil, grease, paint, etc, it is easily replaced and bingo! You now have a new work bench.
Legs and dams: Treated lumber is used for legs in all garage work benches. I like legs to "wrap" the bench top with a 2x6 at each end and middle of the front (and back) and a 2x4 on the side of each end. One picture would be worth a thousand words if I had a camera. 1"x2" pine makes a very nice trim piece for the exposed edges of the OSB. I like to have a "dam" at each corner of any workbence where I will be using small parts and bearings. This dam is located at each corner and will stand proud of or be elevated about 1/8" above the bench top and about 12" long at the front and sides. A dam is especially important if the bench is located near a drain, stairs or a pile of junk. It helps to keep bearings and other small parts from rolling off the bench into a black hole, never, ever, to be found again. Did you ever hear a ball bearing bouncing down 14 steps, one at a time, only to become invisible at the bottom? I have. Ergo the dam.
These are the essentials of what I have learned while building strong, good looking, and frequently copied everyday workbenches. Good luck and have fun in whatever you decide to do. John
