Sneaking up on a school of reds, I can see enough of the fish to get a good idea of how big they are. I'll throw out in front of them, and I'll pull a fish out. The school will take off, but maybe go back to feeding, and if I pull another fish, it'll be about the same size as the previous one.
Now, if it's just a handful of reds tailing, I've seen a big difference in sizes. Might be a big 28"feeding with some sub-20"s or smaller. But in the bigger schools, they seem to be closer in size.
Again, I've only hit big schools a few times. What has your experience been?