The size for a pillow isn't really THAT important: It is just a barrier between your cover (which will reach lower toward the middle) and the water. You do NOT want the cover to be in water as it freezes! It is way more important the pillow's put in the middle of the pool than it's size really.
When it comes to non-round pools, like Woodyp asks, the ''middle'' might be a bit larger, and thus the pillow needed. For a 16'x32' a pillow 4'x8' should do just fine. Again it is more important that it is properly tied up so it remains in the middle than its size. You do NOT want your cover at/below the water line when it freezes up.
Those pillow are, sadly, not that leakproof. You'd rather wish your pool to look like a twinkie than not having the support it needs and you'll be able to see if it is leaking air.
Hope that helped