Don -- It sounds like you may be misunderstanding the concept. On forward and back steps, you can't actually take a step that's a different size than your partner. If the person moving backwards takes a smaller step, he gets stepped on. If he takes a larger step, he separates from body contact. Those who speak of a size differential are usually describing the second step of a turn (eg the side step).
Generally, however, the actual size of differential is very small. For a quarter turn, the size of differential should in theory be exactly the same as the distance between partners. If you're in body contact, that's hardly more than a couple of inches. True, as you increase the amount of turn, you have to increase the differential. So with a typical 3/8 turn, you might be talking about 3 inches. But that's assuming you use absolutely no CBM. And if that's the case, you've got bigger fish to fry.
When you use CBM -- particularly that of the curving variety -- you're essentially diminishing the differential of distance on step 2, and replacing it with a [i]rotational[/i] differential on step 1. In other words, when you curve your track on 1, and the person moving bacwards has a greater curve than the person moving forward, you don't actually need to take a bigger or smaller side step on step 2. Or at least, not nearly as much.
Also, it should be noted that the reason novice dancers don't "get far enough around their partners" usually has a lot more to do with the direction of the second step than the size of it. This is easiest to see in Foxtrot, where the lady literally has no side step at all. This should make it very easy for the man to get around her with very little effort. And yet beginners often miss the mark, sometimes by a large distance. The cause is almost always the man's "deflection" of his second step. Instead of following through his first step and taking the second step in the same direction -- say for exanple towards diag center -- he'll stop slightly short of his first step, deflecting the energy away from his partner and taking his next step towards line of dance. At that point it doesn't matter how big his side step is, he won't have the correct foot placement to move down line of dance on step 3. So the problem here is direction, not distance.
In my experience, I've rarely found a need to speak much about the difference in distance of that side step. It does the job, but I personally think it's overrated, overused and over-emphasized. And those who put too much stock in it usually come out short-changed. Realize that there are more advanced techniques that reduce the need for a size differential to a minimum. And when everything is applied in the appropriate amount, you end up with movement that's more organic and a partnership that's more cohesive.
Regards,
Jonathan