Help on the dimensions of GUI Components

Hello,

I'm trying to improve the appearance of my GUIs and looking for some advice on what size I should make the components in my graphics program. The game is pc standalone and built for 1920 * 1080, and I use GUI.Matrix to scale the GUI components to screen size.

So, say for example I have a button which takes up 1/7 of the screen's width (274 pix before GUI.Matrix Scaling) and 1/10 of the screen's height (108), what size should I make the textures for these button in my graphics program?

Also, is there a recommended file format for GUI graphics? So far, I've been saving everything as .png.

Thanks for reading & your time!