The crush washer is most likely 14 mm nominal ID.

The drain plug is also, of course, 14 mm nominal diameter, most likely requiring
a 17 mm wrench.

To confirm, go to
http://www.hondaautomotiveparts.com/auto/jsp/mws/catdisplay.jsp
put in your car's info, then select "Cylinder Block-Oil Pan."

For the 04 Accord 4-dr DX, they  are  items 14 and 23.

I buy my 91 Civic's crush washer from the dealer, though I think I've heard
Autozone sells them, too.

The dealer shouldn't charge more than about 25 cents for each crush washer. Some
have reported here that having the dealer do an oil change will result in
significant markups for such trivial parts.

Notice that the retail price according to the site above is about 27 cents each.

I just stop by the dealer and pick them up. Typically they are about $1/3.
I just get $2 worth at a time.
It's easier than geting the local chain stores to look up the size. Plus,
you know it's the "correct" part.