The pre-war US regs state that guidemarkers are to be around 18" square, a white field with the regiment's number in red. I know that later in the war US forces seem to have used bunting/printed versions of the National flag and they were around the same, 18" square dimensions. The journal for the Company of Military Historians did an article on this subject some time back. For the Confederates, at least early on they seemingly used silk 1st Nationals as a pattern. Also, North Carolina troops are described as using red flags with the regiment's number in white fixed on them. Some of these are still in the Museum of the Confederacy. Hope this helps...