Honestly, there's no real good spots for big scallops in the Puget Sound.
Most of the scallops you'll find are, at the largest, around a silver dollar. Once you actually get to the hinged muslce, you're looking at a dime sized bite.
I don't know about you, but after cleaning a hundred or so of those and still only having a cup of scallop meat just isn't worth it.
I don't recommend to anyone to harvest the much larger rock scallops. Those are not a plentifuly and easily renewable resource. These are the big scallops fused on rock/cliff faces. One some of the more easily available sites, you'll still see the fused half-shell of rock scallops the size of a dinner plate that were harvest many years ago. Very seldom can you actually find a living scallop that large these days.
Oh yeah, make sure you call the PSP hotline, as scallops *really* hold the toxins...and they hold them for a really long time. For that reason, I won't eat a Puget Sound scallop.
I've heard that the Bellingham area has big weathervane scallops. Only problem is they are relatively deep - start at 200 feet and go down from there.
Feel free to browse my collection of dive site reviews. I don't recall, but I may or may not talk about scallops on some of the WDFW marked sites.
staff.washington.edu/parker/scuba Parker