Is there a named scale for sets such as 75055 Imperial Star Destroyer or 75054 AT-AT
As far as I can find, there are very few official "scale" terms used in LEGO. The term "Minifig-scale" is a fan term and is fairly loose, as the scale of the figs themselves don't really equate to reasonable human dimensions. Add to that the fact that minifig-scale vehicles and buildings have had variable relative sizes compared to minifigs, making it difficult to describe a standard scale that is officially used in LEGO sets.

There are a few "micro-scale" sets, including Micro Scale Space Cruiser, Micro Scale, and Micro Scale Bag End. And, if you will forgive a bit of levity, there is the Kitchen Scale.

My favorite scale that LEGO officially produced was the Midi-scale Millennium Falcon). However, they also did a Midi-scale Star Destroyer that is totally NOT the same scale as the Falcon. So, even in their official terminology the "scale" is a rather fluid thing. 

For a more indepth discussion of scale, they had one over on the Brickset Forums. I particularly like MrShinyAndNew's answer (third one down as of this post).