ORIGINAL: Cribtop
Several discussions in AARs left conventional wisdom as recommending no more than 10ish for good surface combat fighting, although the official impacts are at 15 and 25 as stated. The 10 ship thing is anecdotal but seemed supported by the fact that ships in bigger TFs failed to engage often.
Even more important was TF composition. Matching speeds and, critically, main gun ranges for TF capital ships was demonstrably better per the collective wisdom, articulated best by Nemo's excellent posts on the matter.
I use an even dozen as my optimum target size for a SAG and 15 for my carrier TFs while they operate with two carriers in the early war period and hold to the 25 maximum after I start putting three or more carriers in each CAG in mid '43.
However, there are too few destroyers to go around in the early period so many SAGs operate with 6-8 ships.
There likely is no correct size, just what feels right and performs well for any given player.
Experience is the best determinant.