What size pool cover to get for 18ft AG

I have an 18' AG round pool. When we bought the pool we got an 18' winter cover. The cover barely fits over the pool, especially with the air pillow the cover only goes about 6" over the top of the pool. I have read in the past you should always go up a size when buying a winter cover. What size do you recommend for my pool? Is the next size round pool 21'? I was thinking since the pool is so small and I always keep on top of draining the cover that I might not need the air pillow, it's a pain to keep centered in the pool.

It's been several years since we had our AG pool, but I remember the cover just being big enough to go over the pool and wrap under the edge by a couple of inches all around. The wire thing that came with it to thread through the gromets and the twisty thing that took up the rest of the tension in the wire kept the cover snug under the rim. I don't believe the cover is supposed to "hang down" over the wall of the pool...only be snug under the "lip" a couple of inches. I could be wrong though, but that's how ours was and we never had an issue. As for the air pillow...we didn't use one. We drained the water down about 2 inches below the skimmer and the cover sat directly on top of the water. We just drained the water off the top in the spring...along with all the leaves from the trees in the neighbors' yards...we had no trees in our yard, but had PLENTY of leaves. :mad: