Samoyed or American Eskimo?

First off, the size, as you mentioned. Eskies come in three sizes (in AKC) or two sizes (in UKC and CKC) and the largest of these is only 17" at the shoulder, although of course you'll see the occasional oversize dog. Most are around 16" though, with toys (up to 11";) and minis (up to 15";) being less common. Eskies descend from German and Japanese spitz. They have a tapered muzzle and rounded oval eyes.

 Samoyeds are an ancient draft/herding breed, between 21-25", and have a blunter muzzle. They're not as long-bodied as Eskies and their eyes are more oblique. They've got significantly heavier bone and the pattern of how their hair lies around the head is a little different.

 The German and Japanese Spitz both descend  from some sort of prototypical Nordic spitz, but aren't directly descended from the Samoyed.

 

Eskie's are  much smaller and finer boned that Samy's.  Samy's are much larger than Eskie's (even a big one).  Think Dobes and German Pinchers or Brittanies and Setters.