Anything over 11' is starting to get out of the "Hiking Gear Range" for me anyway, and probably breaks well under that for UL purposes. Do to additional material weight penalties, all the way down the line. I'll bet they lay well regardless.
Tarp can go bigger the better or you could make use of several, if Yakking or Car camping, so yeah bring plenty of coverage. More than you need if you want.
If you're considering going very far afield with that rig I'd opt for using a poncho, if you carry one, or a set of Grizzbeaks, to extend coverage well beyond that of the hammock, in anticipation of possibly severe storms. That would probably save you considerable weight. Once you get beyond 10 X 12's, Tarps seem to jump into ridiculously large sizes. If coverage is the goal rather than housing an outdoor rave, that might be a bit more prudent way to do it.