The smallest grilse I have seen would be about 22 inches and 3 lb in weight. Your typical 2 sea year adult salmon weighs 8 - 13 lb and is 28-32 inches in length. My guess is that you are seeing resident fish (landlocked salmon) that are actively feeding as opposed to searun fish moving upriver to spawn. Then again, they could be fish farm escapees. For Atlantic salmon ID: look for irregular black/brown spots on the flanks (they look like crosses penned in felt tip marker) and a deep V in the tail.
I hope you get to see another one up close :-)
Regards,
Eric