Well....I have never had to measure Nelson for his SMB's - but he stands 16.3hh and is quite thick for a TB *most think he is draft x tb* and he wears a size 100 on his fronts and 200 on his backs. Or, if you are using the new generations, then a medium up front and large in the back.
I wish I could help you - but I say your safest bet would be Mediums or 100's. Get those, give them a try and if they are too small, or too big - you know where to go.
Keep your receipt, and ask the store if you can return them if they are not the correct size.