How did the 10-Gallon Hat get it's name? 10 gallons would be a pretty big hat, wouldn't it?

The traditional ritual that was an invitation to a duel had one man challenging another, and then the challenged one was asked to "choose your weapons" ... to which the reply is "pistols at 20 paces" or some such. This jocular twist on that reply contrasts the "manly" choice of real dueling weapons with the "girlish" act of hitting each other with purses. It's a lame joke, and a bit dated. Make of it what you will. Also, as kids, we would use "creampuffs at 20 paces" in the same vein. You may want to pick a fight, but I'm not going to engage in anything dangerous.

David Letterman's Top Ten; 10-gallon hat; a perfect 10 (sports expression dating back to the 1970s); wouldn't touch it with a 10-foot pole; 10 days that shook the world (Russian Revolution); the 10th Muse (Greek mythology has nine muses, daughters of Zeus. Sappho, a Greek poet of Lesbos, is referred to as the 10th.); Is that a 10-4? (CB jargon for "Do you understand?"); the Ten Commandments; Ten Little Indians; when angry, count to 10 (Thomas Jefferson); 10 days to a firmer figure/happier marriage, etc. (magazine title cliché) and "the perfection of ten" (Shakespeare).

http://www.phrases.org.uk/bulletin_board/8/messages/507.html

Ten denotes completion. Ten completes each set of numbers: units, tens and hundreds.

The Kabbala states about "The Tree of Life, the Ten Holy Emanations
- the Sephira (Sefiroth), the Divine Names".

Hinduism speaks of ten Avatars of Lord Vishnu.

http://www.dlshq.org/religions/esoteric_avatara.htm

Certain styles of cowboy hat have been called a "ten gallon" hat, and it is a common belief that the term arose because a hat could, in theory, be used to carry water. However, the expression does not actually refer to use of the hat as a container. A "ten-gallon" hat in fact holds less than a gallon of water, and a hat that held ten gallons would be unwearable. There are competing theories for the origin of the term, but it is possibly a corruption of the Spanish term tan galan meaning "so gallant", a reference to the headwear of the upper classes (as opposed to the more common sombrero), or a corruption of "galón," or galloon, a type of narrow braided trimming around the crown, possibly a style adapted by the vaqueros. When Texas cowboys misunderstood the word "galón" for "gallon," the popular, though incorrect, legend was born.