When my aquarium fish were 6-1/2" - 7" they preferred FHM 1-1/4" or smaller, they would eat them up to 1-3/4" but the larger ones were usually the last to get eaten. As the RES got larger they would eat them up to 2-1/4" but still preferred the smaller FHM. Length seemed to the the issue, RES like to be able to close their mouth and pop their jaw a few times once they inhale a FHM. If the FHM was too long with the tail sticking out the mouth then they would frequently spit the FHM out.
I do know that the RES in my pond became difficult to catch once my GSH started spawning this year.
