Well, I didn't have a detailed plan when I started this build, so I'm sorry but I'm not going to draw one up now. Nor do I think you need one to actually build this or any other workbench. Take it from me, the less planning you have the better off you will be. But, I do realize that you need some sort of general idea for wood purchasing and a general place of reference. So, here is a generalization of what you need to make this bench.
Legs - my legs are 3 3/4" by 5 3/4" and 30" tall plus 2" tenons. The length depends on how thick the top will be and how tall you want the bench to be. You can buy supper thick stock, or just glue up 2,3,or 4 boards to make the legs thicker. I suggest you use heavy wood, makes the bench heavier and more stable.
Slabs or Top - My slabs are made of 9 six foot boards each. I milled and glued them to make about a 12" slab. Notice I said "about", I wasn't really shooting for an exact measurement, I just got what I got. I made the top from clear Douglas Fir boards that were 2x6 stock (1 1/2" by 5 1/2" in reality). I got lucky and ended up with a 5" thick top, but again, I was just trying for the ballpark of 5" thick, and the boards I started with were very straight and flat to begin with.
And, you don't need to have a split top, you could glue the two slabs together and have a solid top, or have a split top with less or more of a distance between the slabs, your choice just make sure you have a good reason either way.
Once the slabs were done I could start on the undercarriage, because until then I didn't know exactly how wide the top would be. I added 1 1/2" for the split in the middle, a good width for F-style clamps and perfect for a center divider with a 1/2" slot in the middle for tools and such.
Stretchers - All the stretchers have 2" tenons on both ends. The short stretchers are the width I needed for the top, and the long stretchers are 40" between the legs. They are about 1 1/2" thick and 3" wide. The upper short bearing stretchers are 2" wide and 2 1/2" tall, made thicker for strengthening the top. The lower stretchers are all 4" off the floor. I recommend 4" for the lowest, maybe 5" or 6" the highest. You want room for you feet, but you don't need the stretchers to be to high off the ground.
That's just about it. As far as purchasing lumber....If you buy longer boards for making the top you may be able to get enough stock from the off cuts for the short stretchers and even the legs, if you buy 10 foot or 12 foot boards. I had a number of 12 foot boards, most were just over 12 feet and were long enough to cut in half for the 6 foot top. And if you can, buy dry lumber it helps, my boards were very dry and had been stored that way for years. I picked the good straight boards and it helped me reduce the amount of waste. Also, and I really stress this, don't cut anything until you know the measurement or are going to be using the part. You don't want to cut things ahead of time because more than likely the board will move, twist, or cup, or your true measurement will be different. Just start with the slabs and move your way down.
If anyone has more questions about the build, click on the topic on the right and it will bring up past posts, or just ask me a question. I do respond.
PS. if anything doesn't make sense it is because I am coked up on cold medicine and I'm just a little out of it today.