In my shop, I always call them "stems" unless they are stuck.... in which case I call them all sorts of unfriendly names! (The worst ones are the aluminum ones with a cone-style expander. Those things just NEVER want to come out of a rusty set of forks!)
As far as sizes go, most cruiser bike stems are interchangable, but not all of them. Whenever I strip an older bike down for parts, I make sure I keep the stem and seatpost. Of course, being a semi-anarchist, I never measure or label them.... I just toss them into a pile. Then when I need a stem, I root through the pile until I find one that looks cool and fits the bike.
Sometimes whilst rooting through the pile of parts I discover some long-forgotten part like a drum brake, or torrington handlebar... and that starts a whle new wave of creative ideas.