I frequently use drop shadows in my pieces, love to have dimension in my work. The shadow width depends on how high you want you blocks to float and how low the light source is. Skinny shadows imply the light is overhead, wider implies lower.
A good place to start is around 15% of the size of your block, rounded up to make it a sensible measurement for cutting and piecing. There are no rules and making adjustments to wind up with the final size you want is completely logical.
Some examples:
One hint to watch out for...the outer border on the two sides with shadows can be confusing. Remember that you want your eye to center the blocks, Not the shadows, it's all to enhance the illusion. The outer edge of the block to the edge of the quilt should be the same all the way around, regardless of shadows. Have a grand time!!