How do I find out the size of my computer screen?

The size of a desktop computer monitor is determined by physically measuring the screen. Using a measuring tape, start at the top-left corner and pull it diagonally to the bottom-right corner. Be sure to only measure the screen; do not include the bezel (the plastic edge) around the screen.

What are the dimensions of a 15.6 inch laptop screen?

13.6″ width (side-by-side) seems about right for a 15.6″ diagonal screen.