Looking to buy a diesel pusher - How big is too big???

A few states have, or at least recently had, a 40' limit on legal motorhome length. In practice I doubt you'd be cited for a 45' rig anywhere unless you were doing something ill-advised with it (i.e. going down a road where you're clearly too long for the conditions). But, if you want to be legal anywhere in the US, you should probably limit the length to 40'.