Question: Font size and pixels

What is the relationship between font size and screen pixels on plots? My screen are 2560 x1440 and 1920 x1200. Is for exanple size 12 font 12 pixels high?

Also when a diplay plot is say 800 x 600 pixels in size does the plot  does it use the max horizontal or vertical number of pixels? I know it may not fill the plot area in both directions. Or are some pixels trimmed of around the sides. Is it that the bounding box is the 800 x 600? I know when a caption is added the plot area is reduced in size vertically. 

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