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.
Sample range is the number of slightly offset shadow maps created to soften the shadow edges.
The higher the number, the softer the shadow edges. High Sample Range settings can slow shadow calculation significantly. The settings are all affected by each other, so if you change one, you usually must adjust one or more of the others to compensate. For example, a high Size setting overrides the effect of the Sample Range setting. You will generally be more productive when using Shadow Maps, but the Raytrace shadows can sometimes be more convincing in outdoor scenes.