I've come to love building workbenches from 2x4's. They're (reasonably) cheap, soft enough to work easily, and you can essentially use mass to compensate for intrinsic rigidity. Maybe add plywood shear panels in strategic places. Even 1/4" plywood shear panel will beat most brace structures.

2x4s are cheap enough to try a design for a while & throw it out (or downgrade it to patio furniture) if it doesn't work. Soft enough to just plane a millimeter off the top when it gets beat up. If you invest in expensive hardware, move it to the new iteration. I've got like three of them now.

A lot of the dimensions came from the limitations of the material I had on hand. A friend gave me a stack of 2x4s, and I processed them to get the most I could out of the boards for the top. So the thickness, length, and width were mostly dictated by that. I tried to maximize the size of the bench since I often work with large material. Case in point, first project was some sound panels that were the same size or larger than the top.

For the design, I watched a ton of videos and tried to pick out the best parts. It was important to me to have the legs flush with the edge of the top so I could work with material vertically ans clamp to the legs.

The bracing I commented on elsewhere, but I wanted some extra structure, some extra weight, and dog holes on the working side.

I didn't feel the need for fancy joinery, but I wanted the strength so I used half laps everywhere, which still turned out to be a challenge with the size of the parts.