As far as the grid goes, I usually try to not make any single scene much bigger than 3x the size of it. Otherwise, you get a slower frame rate. When actually modeling a scene however, I usually eyeball everything. When I'm done, I simply group every single object into one group and scale the group size down to whatever size I want. You don't want to include some thnigs in the group when if you want to scale it down, though like clusters for instance. If you scale those down I believe it will give you undesirable results in the end. And I consider the grid as "units."