Cameras and GUIs

I have a 2D texture that I'm going to use as a buttons "up" state. I'm using EZGUI but I might as well be using a flat plane to display it, it doesn't seem to make any difference.

I can't seem to to get the texture to display at it's original size no matter what I try. Camera distance is only complicating the matter, how far, too close, the field of view etc. Are there magic numbers involved in getting a button to display properly on screen?

I'm using a perspective camera but I had the same issue with an orthographic one, the size slider doesn't make any sense to me. And I saw a post saying that the orthographic cameras field of view is half the height of the viewport? Whaaa?

I'm hella confused on how camera views work in relation to GUI elements, and the unity manual on cameras doesn't exactly help either.

I dont know if you use this one but look at the answerthatmaybe what your looking for