It can get about 6' wide. Of course, if you place a new plant three feet away, it would look odd until it grows wider. You should also consider if your house foundation leeches lime and makes the soil alkaline too. That might force you to separate it a little more.

Thanks. Has anyone had issues with paniculatas being near foundations, in terms of lack of acidity? My soil is pretty acid (my ES are blue without any supplements).

If it's not a freshly (3-4-5 y.old) built house leeching from the foundation shouldn't be a problem, however you should put center of the plant AT LEAST as far away from the wall as far is your overhang extends + 8-12". I'd say (for the Limelight) 4' away from the wall would be about proper distance.

I've always read that 2' should be left between the house and any shrubs - to allow for air circulation to prevent rot, and also to allow room to paint etc. So if the bush gets 6' wide, OC should be 5'.