Joanne, at one time I made pet clothes for sale. I noticed that there are very few things available for med. large, large, and extra-large size dogs. Most are for the teacup or small sizes. You may have friends or relatives with larger dogs that you could use as models. Then list those items separately. Most pet clothing shops cater to the smaller pets, so yours could stand out. Also, if you did that, make sure that your tags mention the larger sizes.
One more thing, I was surprised by how much extra fabric it takes to make an item for a larger size dog than a small one. It isn't that way when sewing for kids. By separating the sizes into individual listings, you would then be able to adjust the prices to cover your costs. Otherwise you might lose a lot of money.
Another thing I just remembered, I had purchased size labels for the different size pet outfits, and it was worth the little extra expense. I had sizes from XXS to XL. When you have a large inventory with different sizes of similar items, it can get confusing finding the correct size if they aren't marked. Good luck.