I've been there a number of times, and it IS a big store. They've got lots of stuff in big scales, probably more in HO than anything, a little bit of N and no Z whatsoever. The selection is great as long as you're not looking for anything erudite.
As far as being the "biggest store", I couldn't say. Lots of train stores run ads calling themselves the "biggest store." But it's definitely the largest store in the metro Boston area - I've been to them all.
Also, check the map closely. That neighborhood has a lot of one way streets, and actually getting to the store is a bit roundabout from the main drag.