how you know how big is the mesh?

I want to know how big is the mesh when in gird units that display the numbers. I can’t seem to find it.

In object mode, the DIMX/DIMY/DIMZ fields in the ‘n-key window’. Only for whole objects or groups of objects. It’s a basic ‘difference in x between largest and smallest for all verts’ sort of thing, or, a bounding box.