Keep in mind that even if you match the pool size, things may not line up quite right. I have that issue right now. My new 27' does not slot into the deck my old 27' was in quite right. My deck goes around about 40% of my pool. Part of that is because the new pool has a higher wall, but part of it is also pools have different leg sizes and configurations, and it is a lot easier to build a deck up to an existing pool, then to try to build a new pool tight to an existing deck.
It is very likely that if you need modifications made to the deck, that your pool builder will not do it (or at least won't do it well). They are pool installers, not carpenters. When my installer looked at my existing deck he said "we will cut if we have to, but we don't cut pretty". I was fine with that because I had some plans in mind to change how the deck met the pool anyway.
Still, I would go with the same size that was there. Composite flexes more than wood, so it is harder to extend it to the edge of a smaller pool without support.
