How much of a different in size are Border Collies and Blue Heelers, and how big are they compared to adults?

I may be adopting a Blue Heeler or a Border Collie, or any medium sized Aussie dog. If you can think of a good small-medium or medium sized Aussie dog, that would be great, but I want to know how big Border Collies are to Heelers and how big they are compared to a normal adult.

I have border collies and they come up to my knees.

Both breeds you mentioned (Australian Cattle Dog, Border Collie) are working breeds and so have a wide variety of sizes and types. I've met ACDs (also called Blue or Red Heelers) that were 14" tall, weighed about 30lbs and were quite stocky, and ACDs that were 24" tall and weighed 70 lbs. It depends on the dog. Similarly in Border Collies where I've met dogs ranging in size from 25#-80#.Both breeds *tend* to be on the smaller side of medium for a dog... about the size of a Springer Spaniel would be typical, or their back would be slightly lower than the seat of a standard dining room chair.

Why not Google the breed standard for each of these breeds. It's an Australian Cattle Dog, not a Blue Heeler. Unless you have quite a bit of money, I would not suggest a Border Collie. As you know, Border Collies were bred to work, and need to herd. In order to do so, you will need to leave the BC with a sheepdog trainer for 3-4 months, at an average cost of $500.00 a month. After that, you will need to continue to take lessons 2-3 days a week, running from $35.00 to $50.00 an hour, depending on the caliber of the trainer. My national trainer charges $500.00 a month for in house training, and $50.00 an hour for lessons.