You aren't in the wrong forum or perhaps the hunting/fishing outdoor forum might be better. It is all personal preference really. To me, with fish, the blade length would be consistent with the typical fish size you are likely to catch. I wouldn't want to choose a 3" blade to use on a 8 foot shark if I could help it, but on the other hand, unless you just want to carry or buy a lot of knives, I might make do with the 3" blade as it might be once in a lifetime kind of thing on the shark. On the other hand, a 7" bladed knife is really clumsy trying to clean a squirrel or rabbit but it can be done. But you choose a size or length that allows you to be in control of the blade and do the kinds of cuts you might do. I don't clean many fish personally (Catch & Release) and I probably would have a SAK with me for the size of fish I catch. If I were cleaning a lot of pan fish (crappie, bass, etc.), I'd want a longish thin blade or in other words, a filet knife. Plus all that fish gunk gets inside the sak/folder and you have to clean it out.
The blade design/shape may be more important than the length within reason. Clearly a fixed bladed knife is a better choice overall, but I generally will only have a folder with me. If I am hunting, I will frequently have a fixed blade with me and for deer sized game, the bird and trout sized knife is really all you need and mostly all I want. The common preference is for a fixed blade up to about 4" long as it is a size that for most of us it is easy to manipulate and use safely. Whether you like a pointy blade shape or a less pointy type blade, is up to you, I generally like a more pointy blade overall and typically shaped like the KaBar Becker BK-15, but the BK-16 (drop point) works well for most.