I may be speaking out of turn here and let's hope some of the better informed experts will chime in on this. At the opening of the war a lot of artillery batteries, as well as other units, carried some rather distinctive unit flags. Some were given to them by the townsfolk from where their unit was raised, some were made by some of the officer's wives, etc. By Gettysburg, most of these had been weeded out and sent home for safekeeping due to the attempt to standardize the flags carried by the Army of Northern Virginia. Provision was made for an "artillery sized" ANV flag (being 36"x36" square) but in actual practice, few were issued. Most ANV artillery units carried an infantry sized (48"x48" square) flag when they carried a flag at all. Hope this helps.
Charles Brown