Many shadow casting lights can use all the available RAM and cause the computer to page to disk during rendering, thereby reducing render speed to unproductive levels. Use shadow casting lights only when necessary and only use shadow casting Omni lights in very special cases, such as flickering candles or bare light bulbs. Shadows from Omni lights use more than six times the computer resources when compared to a Spot or Direct light. Note that all lights in VIZ have shadow casting on by default and you must get into the habit of clearing the Cast Shadows checkbox. Shadow bias moves the shadow towards or away from the light source.
Smaller numbers move the shadow toward the light, larger numbers move the shadow away.
For scenes with large objects, such as a building site, you can often get better shadows by checking the Absolute Map Bias checkbox. Shadow size is the resolution in pixels of the grayscale bitmap composited into the final rendered image. The higher the resolution, the crisper the shadow edges. A good rule of thumb is to set the Size to the largest resolution of the final output image. For example, when rendering to 1,024 x 768 images, use a size of 1,024.