"Just how much will a copperhead hold?"
Anywhere from a few ounces to a few thousand pounds. Experience will tell. A good rule is that anything that is larger than a #2 in good rock will hold a lot, including falls. A number one might hold a little fall, and a number 2 will hold a small fall. All of this is super dependant on the actual placement, but if I place it, those would be the rules I would use in good rock.

"What size is the most used on a wall?"
Probably the number two, followed by the number 3.

"What is the deal with your EQ Copperheads?"
The idea is to be able to slide the floating sleeve up or down the cable and then place this sliding sleeve as an additional piece in concert with the standard head. The added grip just might be what you need to get past an especially tricky section of crack.

"Should I get copper or aluminum?"
Copper can be used more than once if placed with care. Aluminum is so soft that using them more than once is not such a great idea. The thing is, aluminum will hold better in some cases. So, usually you will want to take some of each to cover all your options. We use aluminum in the larger sizes to take advantage of the softness of the material as copper would just be too hard to get to conform to the irregularities of the crack.

"Can I place them like a stopper?"
Yes but why? This is not their intended use and there is the chance that you could pop the head off the cable if you fell onto the copperhead while using it in this way.