Calculate size of the mesh OR mesh behave like a fabric

Hi! Question: I’m transforming an image (translating and scaling the UV) but i need to show the units in inches, you know the UV units (0-1) so i use the bounding box to calculate the size of the mesh, this calculation is an approximation to the size, the problem is the irregular shape of the geometry, then the result sometimes differs to much depending on the shape.

The questions are:
-Is there a way to calculate better the size of the mesh?
-I could put the image in a 1x1 mesh and transform it. But: How to make the mesh behave like a fabric (copying the shape it is on) and this only can be moved over the other mesh (As if it were magnetized)