A tall workbench is good for detailed work, cutting joinery, and for power tool use. 34″ – 36″ (86cm – 91cm) tend to be the most common workbench height for woodworking. A height in between these two extremes tends to be the most popular, particular if you do a range of activities at your workbench.

How flat should a workbench be?

Benchtop flatness seems more important for hand work than for machine work. If you work mostly with power tools, I'd say a 1/16″ cup down the middle is fine. Your machines (a planer and a jointer) will help ensure your wood is flat. You'll mostly be clamping your work to your bench to rout it, biscuit it and so forth.

How thick should a workbench top be?

The thickness of your bench top needs to be about 75mm. That's 3” in imperial. I would say that the bench top needs to be a minimum of 63mm or 2 ½” thick if you have stock already or that's a size you can find.