I replaced my filter this spring at the start of the season out of necessity. Mine was leaking, falling apart and the valve was shot. I have a 1HP, single speed pump, 30K gallon pool and the old filter was a Pentair 21" sand. WAY undersized. I elected to go with at S270T for a few reasons, despite it being undersized for efficiency and filtering ability. I really should have a 30" plus. However, the largest filter that would fit in the footprint of my concrete slab between the pump and the heater was the 27". I also have a short swimming season here in MN and water is relatively inexpensive, which means I may have to backwash more often due to a smaller filter and I may have to run my pump longer to achieve desired filtration. I haven't had to backwash since SLAMing my pool in May. Running my pump 6-7 hours per day keeps my water sparkling, with some floaters by the afternoon but can't really be avoided. I also don't get alot of pollen or leaves, just mostly bugs and dust for me. I would have gone larger if it made economic and logistical sense but it didn't for me.
Cliff notes: Larger is better. Smaller can work fine, but there are downsides. I think 27" would work but you may be happier with larger over long term.
Side note: I probably have the same old, beat up heater you have. It works... barely but I don't use it.
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