Double your original length if you can and I wouldnt go anywhere less than 30" deep. Most countertop is 30"(give or take) deep to give you an idea of how deep you are talking.
If you only build 40" wide, you WILL be building a second bench in short order. The bench we built at my dads house was ~60" and its PACKED FULL and we wished we had built it another foot or so(and there was room to do it too).
We have installed a center support for those lower shelves since they were bending pretty good once all loaded up. They just didnt have enough girth to hold the load.
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