Personally, I would not use that type of carry, but I'm not saying it's wrong, just not what I would do. Here's why - Once you get them out of the water, they lose their buoyancy and you have a lot of dead weight. That dead weight can pull you backwards and over because of the leverage factor. Think about how you would tow a tired or unconscious diver toward shore, backwards and under the arms. Use that same position to drag them out. Use yourself and your weigh for leverage, not theirs against it.
As for different sizes, dragging will cover most combinations, but won't be the most expeditious. Flyboy08 recommendation of the fireman's carry is the most expedient, but not an option if you (the rescuer) are much smaller, tired, etc.
Just as you plan a dive, plan a rescue. Think ahead of time about what you would do with your particular buddy, or every member of your dive group.