Nope, "steamers" isn't a size designation. Soft-shell clams don't have a size designation.
Not that I'm a big fan of Wikipedia, but they have a pic & some info on soft-shells that might give you an idea:
Soft-shell clam - Wikipedia, the free encyclopedia
Also - if any of you have enjoyed "fried clams" in a restaurant or from the supermarket frozen food section, those are soft-shell clams.
I fondly remember clamming for soft-shells growing up on Long Island, NY's waterfront. "Razor Clams", though different in appearance, are also considered a "soft shell". Back when I was growing up (when the dinosaurs roamed - lol!), we used to toss Razor Clams back as they were supposedly "inedible". Little did we know. They're now a much sought after prize.
Oh, & one more little educational tidbit - the HUGE Geoduck of the Pacific Northwest is a soft-shell clam!