Cooper can wear a 4T kid's shirt, as long as I cut out a U-shape from the bottom as a pee hole. Look for sales on the "tagless" type of t-shirts, as I think they would be much more comfortable.
He is a little bigger than most here at 28 pounds.
Cooper must wear at least a t-shirt under 60 degrees F. Otherwise, he shivvers and scratches. Under 50, we generally break out the fleece hoodies.
I make the hoodies myself, as a single or double layer of fleece. It took a few tries (and it's still a work in progress) to make a pattern that works for him. I actually started with little vests, but he grew out of those in his first winter! Since our fur kids are relatively small, go to your fabric store and look in the remnants bins. They usually have some great fleece at a great price. And fleece is pretty forgiving, since it doesn't unravel too easily. The hardest part is figuring out how to make a closure that is easy to work when you have a wriggling dog!
I might have photos in my gallery. Otherwise, I'll try to post some of his outfits.