What is the maximum DIE_AREA allowed for MPW-3?

Same as for MPW-1 and -2.

I wasn't at any of that MPW. If you provide me the place where is detailed I will really appreciate it. I have seen the MPW2 video and I think I heard 17mm2 for the user area? It could be with any combination of width and height?

If you clone the caravel or caravel_user_project repository, the "user_project_wrapper" cell is the area you have---it's an exact area, with pins (labels) in specific locations. The area is a bit over 10 mm^2. (The 17 mm^2 probably refers to the whole chip, not the user project area). I forget the exact width and height, but it's close to 3mm wide and 3.5mm high.

Where is that file that tells the cell area of the user project wrapper? The default values of config.ctl of user_project_example are 0,0 900, 600. I suppose that I can write safely 0,0, 3000, 3000 as in the documentation of openLane says that the units are in um. That's right?