|Tara demoing on a wooden platform|
|Larger size foam platform|
In general the length of the box should be 3-4 inches longer and 1-2 inches wider than your dog stands when relaxed and comfortable. The box should be no more than 2-2.5" inches in height and should be some sort of non-slip on both sides -- so that it doesn't slip on your flooring and so that your dog doesn't slip when standing on it.
Or, if you want you can order them here: http://cdf-freestyle.com/platforms2.htm
Or here is a video on how to make your own fairly easily: