Size of Flags
Although flags come in a fixed, standardized series of sizes, there are
guidelines which will help in selecting the proper size for your boat.
Keeping in mind that flags are more often too small than too large, use
the rules given below, and round upward to the nearest larger standard
size.
The flag at the stern of your boat-U.S. ensign, yacht ensign, or USPS
ensign-should be one inch on the fly for each foot of overall length.
The hoist will normally be two-thirds of the fly, but some flags such as
the USCG Auxiliary ensign have different proportions.


Placement of flags
The U.S. ensign is proper for all U.S. yachts, without reservation. This
is "Old Glory," with 50 stars and 13 stripes. All boats, when at anchor,
fly it from the stern staff, if so equipped, only while occupied. It is
flown from the stern staff of powerboats underway on inland waters. If
the powerboat has a mast and gaff, the proper display is at the gaff. On
a sportsfisherman, where a stern staff would be in the way of the
action, the practice is to fly the ensign from a halyard rigged just
behind the tuna tower.
On Marconi-rigged sailboats under sail alone, the practice for many
years had been to fly the ensign from the leech of the aftermost sail,
approximately 2/3 the length of the leech above the clew. This puts it
in about the same position it would occupy if the boat were gaff-rigged,
and on gaff-rigged sailboats it is proper to fly the ensign from the
peak of the aftermost gaff.
The advent of the modern high-aspect-ratio rig, with the boom end well
inboard of the stern, has made it is possible to fly the ensign from the
stern staff of a sailboat underway, and this is now an accepted
practice. However, the ensign should never be displayed while the boat
is racing. Under power alone, or at anchor or made fast, the ensign
should be flown from the stern staff of all sailboats. If an overhanging
boom requires that the staff be off center, it should preferably be on
the starboard side.

http://www.deepcreekyachtclub.com/We....htm#us-ensign

Wilbur Hubbard