I'm had some fischer screws a while back and wanted some more, but not to sure of the size or how to work it out.

any idea what size screws the picture below is?

Thanks

Bog standard 4x50 like any other.

How do screw sizes work? whats the 2 sizes? example, 4.0 x 50mm?

also can be in inches ie 8x2"

Using 4 x 50 as the example, the 4 is the diameter of the shank just below the head in mm and the 50 is the length of the shank in mm not including the head.

If you still work in old money have a look here http://www.thesitebox.com/Page/21/Metric-to-Imperial-Screw-Size-Conversion.aspx