I agree that sticking with the tire size specified by the manufacturer is best. Have you checked the tire sticker on the door to confirm the correct tire size for your Caravan? Stick with that size. I'm not sure of the theory that going with a larger is better. Either wider or larger diameter could lead to rubbing on turns or on large bumps. A rubbing tire at speed could be a safety issue, although the differences you're currently talking are not likely to cause a safety concern.
-Rod