As a general rule I make the panels about 3/4 the thickness of the styles. However, it to some degree depends on the moulding. If the moulding is not very deep, I adjust the thickness of the panels accordingly. You want to create shadows from the moulding / reliefs to create interest / feeling of quality. The easiest solution to see what you like us to toddle off to a shed / door retailer and look at panelled doors and see what you like. Measure up and copy the dimensions.