Looks like others have used melatonin with dogs as well:
http://www.canine-epilepsy-guardian-angels.com/Melatonin_info.htmFrom their site:
Generally a dose of 3mg is appropriate for a 35-100 lb dog. Smaller dogs get 1.5mg, and larger dogs may get 6mg.
That's pretty comparable to a few other links I ran across, so if you use the rough 35-100 pounds (18-50 kilos) for the 3mg dose, and under 30 pounds (15 kilos and under) would get half that...1.5mgs. You can probably slide that scale a bit if you're inbetween on mgs of melatonin or dog weight.....but that should give you a rough idea.
PS...let us know if it helps!