If the icons on a 15' by 15' pc are 3/8' by 3/8' and i need them to be proportional to a 6' by 6' chart

Question

How large should they be on the chart?

Answer 1

It appears that you want to perform a linear scaling of all objects from a 15'x15' space to a 6'x6' space.  Since the spaces are the same proportion (square in this case) then the scale factor will be the same in each linear direction: 6'/15' = 2/5.  Using this scale factor on your icon results in a size of 2/5 * 3/8' = 6/40' = 3/20'.  So the size of your icons should be 3/20' x 3/20'