GUI units (pixels?)

Hi,
i'm trying to build a layout of a UIscreen with a exactly size in pixels (1024x576), but it seems like the units in Unity are totally different than pixels... so, how can i work if i need an specific size for a mobile screen output?
What i did was export a .png with the exactly size and put it as a base of the Canvas so i know that that size is the correct one, and the start to build on top of that. But of course, once i try to fit the visual area of the camera it works but the measures are not in pixels so... kinda weird!
So... how it works? Thanks in advance!