Actually most of the early fish in the river are bound for the Willamette. That is why fishing is open below I5 right now, to target the willamette fish. The run on the Willamette is projected to be 73,800 this year which when combined with the better water conditions should make this a good year.
Last year there were loads of fish in the system, but there was virtually no current to get any fishing done. Some people had terrible years on the Willamette last year
I believe of the tribs the Willamette gets the biggest run, however you have to look at the size of the river, I think in fish per square mile the wind is the most productive around. With it's liberal limits and high fish density it turns into a white trash fest pretty quickly though.
As far as I know the biggest springer caught was on the Clack back in the 80's don't remember the specifics, but I believe it was over 50#. That fish was a freak of nature though. I think there is very little variance between average sizes in different tribs. However someone could definately prove me wrong.
Hope this answered some of your questions.