I dunno if it's a good coat, but you size it according to the information right under the order chart:
"Important Information:
Sizes: X-Small 10"-12", Small 13"-16", Medium 17"-19", Large 20"-24", X-Large 25"-29" "
For dog clothes, that size usually refers to their chest girth, which you measure around the chest right behind the front legs, like this:
I couldn't find a diagram that looked like Scooby
Some coats measure around the neck too, but it looks like that one's adjustable, and Scooby doesn't have an unusually thick or thin neck for his size, so I wouldn't worry too much.
Also, saw
this coat the other day...was considering it for Kim until I realized...she's not cold lol. I think you have to buy a minimum order $$ amount from that site, though (I think I saw ilovegreys saying something like that), and I don't know what that minimum is.