I suppose one of the first questions is what's ones definition of a smaller ship.
The "smallest" ship we cruised in Alaska was HAL's Zaandam. Awesome.
I think Celebrity would be a good choice too however they don't go to Glacier bay and I'm of the opinion that, especially for a first Alaskan cruise, Glacier bay is a must. So I'd say look at HAL. A bit older crowd, smaller ships, great service.