All sizes are approximate and depend on the temple structure (a tall narrow temple will be taller than a temple that is small and square.) Christmas Ornament is equivalent of a small glass temple in a 3" glass ring with glass loop at top for ribbon or hook to hang. The medium and large sizes are a popular choice as a memorable wedding cake topper.
For the Mesa Arizona temple the dimensions are more or less as follows:
Small: width-2" length-2.5" height-2"
Medium: width-3" length-3.5" height-2.5"
Large: width-3.5" length-3.5 height-3"
These dimensions are all approximate and aren't exact for each temple Marge creates. These numbers are just to give you a good general idea of the size of each temple and NOT to give exact measurements.