You could place an image of a ruler as the backdrop to each viewport and size it to match the correct dimension.

I don't really understand the need because in 3D modelling size is either very specific, or can be relative and changed. So if I am making a real car I will be absolutely specific about size. If I am making a model car I can size it at perhaps one tenth of the size of the real thing and change my mind at any time and scale it up or down. If I make a vase, proportions may be more important than size because it may work well at different sizes.

If you want the grid to match your monitor you will be modelling small things and what about zooming in and out for precise placement?

What is your use-case for this?