The optimal size for your decorations depends on the size of your yard. Homes with large front yards can use almost any decoration. If you have a small front yard, decorations more than 36" high may start to overwhelm the space or block windows.
If you live somewhere subject to high winds and snowstorms, remember that large and lightweight decorations can be blown around and become hazards. Make sure to properly secure decorations or bring them inside the garage if the weather calls for bad storms.