When is a mecha a robot?

About a quarter of all the robot posts are also taged mecha. The robot wiki says to use mecha or robot depending on size. Now I know some of the images has both robots and mechas but they are not many. Can a mecha be a robot by Danbooru standard and if yes when?

If no how do we differentiate when there is nothing to compare for size?

If I understand the robot wiki correctly, mecha shouldn't be tagged as robots. The robot tag is used here for "other" robots that don't fall in the listed categories (android, cyborg, etc.) and none of the listed categories seems to imply robot.

So if there's no reference to compare sizes, and mecha are giant robots, then they are tagged as robots in this case.

I thought that robots are always unmanned and don't have a cockpit for a person to control it..

Transformers can be mecha too.

Transformers seems to always be mecha even if they are smaller than humans. One of the reasons I asked this question.