Maximo wears the Buddy Belt and he doesn't get mats. He has a full length silky coat, so I can't say how a cottony coat would fare in it.
As for size, I would order the larger one. If you order online, look for an exchange policy. I believe Muttropolis will exchange an un-used BB if you aren't happy with the size or fit.
Maximo fits in his size 3, but I'm going to order him the size 4 because I think it would make arranging his leg "feathers" easier, and the larger size would fit over clothes as others pointed out. (Plus, we have another Yorkie coming who will fit in the old 3
)