I am asking a few questions so I can better create or reproduce current flies in my stock and maybe offer some up.

I know the primary forage for sandies, hybrids, and stripers in most lakes in Texas is the ubiquitous threadfin shad but I also no other forage is taken fairly regularly. Gizzard shad, ghost minnows (inland silversides), and tail spot shiners are on high on the menu too. My question would be what size do these baitfish present themselves in most often when the fish turn their minds to them.

For instance during the Spring spawning runs white and grey #6 clouser minnow, approximately 2 1/2 inches long really does the trick for mimicking the tail spot shiners in the creeks. Using live bait under lights at crappie houses for sandies I have seen the fish ignore the clouds of 1/4 in to 1/2 in baby threadfins they are gorging on to smash 4 in ghost minnows. I guess I am asking what sizes of baits do you prefer in what season and are there any other forage fish (or anything else I should be considering.

I don't know that size matters more than looking more vulnerable / easier to eat.
Using your example, maybe the sandies were going after the ghost minnows because they were moving a little slower or the minnows were closer to the surface making them an easier target for food?

I've had to catch Ghost minnows to match the hatch when fish are targeting them at Green lights. I've also seen stripers at Texoma ignore my larger lures when they were focused on big schools of dime sized shad. If you are using a fly at lights at night it's going to need to look like one of those ghost minnows.

Match the hatch, fish will almost always eat what is there. Typically small baits in late spring and summer and bigger baits in fall winter and early spring. Striper, sandies, hybrids, and yellow bass eat whatever forage is present, they’re not only Shad eaters, in rivers they often eat crawfish that get dislodged when floods occur too. I’ve found sandies in the creeks in the spring will eat just about anything you can net in the creeks from tiny red shiners, to ghost minnows, spottails, golden shiners, and they absolutely LOVE small bluegill that time of year if you can net some. I’ve fished rivers where I’ve caught hybrids on bluegill, small suckers, and assorted other baitfish that I was able to net.