As a combat application 'Mechs are just pure fantasy. But on civilian side up to a certain scale there well might be some utility for walking units. Roughly akin to BT's IndustrialMechs or Alien's style loaders. But terrain movement would still remain unlikely - real problem in addition to the actual mechanical reliability of the knees and such would be the weight, or more precisely the ground pressure. After all there is a reason why tanks have long and wide tracks. Whole another thing is that you really, really would not want to have things as tall as flagpoles. They would be just instant targets and trouble magnets without any benefits. As said before smaller applications - much more akin to powered armor or such - would seem more likely.
As to the question by the OP... I kinda agree with one of the previous posters in that assuming 20 t being around 6-8 meters tall is a good way to start.
As to the question by the OP... I kinda agree with one of the previous posters in that assuming 20 t being around 6-8 meters tall is a good way to start.