Border collie average size?

We have a dog that is overweight, he's most likely a border collie, maybe a shetland sheepdog {He looks like a border collie but his coat is more like that of a sheltie} When we brought him home the animal shelter he was listed as an american eskimo mix and I am pretty sure the vet still has him listed as one and I don't know what the size differance between the three breeds so I am just wondering, what is the average weight for a border collie?