The right height
The more research performed on this, the greater the variation you will find. Having made lots of workbenches and worked on lots as well, the information here is our opinion on the matter.
For the hybrid woodworker (combination of power- and hand-tool work), the best height is the crease in your wrist to the floor. For an average height person that is around 900mm.
If you are using only hand tools, add 50mm to this height. This is because you need the power through your hips and legs when hand planing and to be closer to your work.
If you are using only power tools, subtract 50mm from this height. This is because all of the power is coming from your tools.
The right depth
The right depth is as far as you can comfortably reach. Any more than this and the part that you can’t reach will become an area that just collects dust and unused tools. For most people, the right depth is around 600mm.
If the workbench is to be used the centre of your workspace and you can work from all sides of the bench, possibly add 150mm.
The only caveat to this information is if the bench is to be used to assemble large pieces of furniture. If you are making large pieces of furniture all of the time, then go large. If you might make some large furniture from time to time, find a way to extend your normal workbench. Use a sawhorse and a panel of plywood.
The right length
This is the easy one. Make the workbench as long as you can comfortably fit in your workspace.