rule #1 for razor clams on the central/south coast- the clams available to dig (exposed at minus tides) tend to be concentrated near things that stick out into the ocean- north and south jetties at river mouths and also the natural cliffs extend into the ocean. this is due to the broadcast spawning of the razor clams that has them swirling around at the mercy of the currents.
that doesn't mean all these beaches near jetties will always have lots of large clams but if you find mostly small clams at one, try a different one the next time and come back next year to the one with small clams this year. cheers, roger
FYI- big river/bay easy access areas- CoosBay south jetty (Bastendorf beach), Umpqua south jetty, Siuslaw south and north jetty, Alsea north and south, Yaquina south and north, plus some smaller rivers and terminal beaches at north and south sides of points/heads/cliffs.