I fly the ensign from the truck when underway. Things are going really well when I can "shift colors" and pour a cup of joe and still be within 100 yards of my mooring. Cotton line works well for a halyard. Synthetic is too durable. If you do foul the halyard, you can easily cut it or wear it out by chafing it. Cotton doesn't break down in UV like synthetic either.
The best size ensign to fly is one you can reach the bottom of when hoisted.(!) The No.2 Navy ensign has a luff of about twenty feet and if it doesn't work out you could cover your boat with it. You could cover your garage with a No.1. Seriously though, you may have to do a little engineering at the truck, and run the halyard all the way aft. It's where it's supposed to be flown, and it sure looks handsome going by.