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Size, hammer, and action aren't necessarily related.
Someone else can correct me if I am wrong, but I don't know of any autoloaders that have shrouded or otherwise non-exposed hammers. When you look at an auto that doesn't have a hammer, you are generally looking at a striker fired gun. Basically, a striker is kind of like the hammer and firing pin all in one---the trigger pulls the striker against a spring, then releases it forward to hit the primer. No separate hammer involved.
Striker fired guns will basically have the same trigger pull every time. A hammer fired gun may be double-action only (DAO) and have the same pull every time; it might be DA for the first shot and SA for follow-up shots; it might even be cocked-and-locked single action only (SAO).
Glocks are striker-fired, and may be small pocket-sized guns or full sized. A SIG P239 is a compact gun with a hammer that can be either DA/SA or DAO (actually, a SIG-specific type of DAO called DAK). The SIG P238 is a pocket-sized, hammer-fired SAO gun.
The main difference between striker-fired and hammer-fired guns is that DAO or DA/SA hammer-fired guns have second strike capability; that is, if the first trigger pull fails to fire the round, you can immediately pull the trigger again and try to fire the round a second time. Some people think this is important because the vast majority of quality factory ammo that fails to fire once will go off if hit a second time. If you have a striker or an SAO hammer gun, once you get the dud you have to do a full tap-rack-bang stoppage drill.