What is the relationship between screen physical size and canvas->Clip?

Hi… I am currently making a game for mobile phones.

I am currently confused about one thing. How do I scale the HUDs.

I have currently written a lot of codes which scale the HUD depending upon the Canvas->ClipX and Canvas->ClipY .

Canvas is the screen resolution? Right?

So if a phone has low physical size and higher resolution something like iphone5, wont the HUD scale really low? Is there any way to finding the physical screen size?