The Army of the Potomac brigade flags were triangular, 4'9" tall on the hoist, and the other two sides were 6' long. Two of the brigade flags in the New York State collection are made of wool, but I can't say that all of them were wool.

http://dmna.state.ny.us/historic/btlflags/other/1stBrig1stDiv3rdCorps_DesFlag1995.3316.htm

http://dmna.state.ny.us/historic/btlflags/other/1stBrig3rdDiv6thCorps_DesFlags1.htm

The 3rd Brigade, 2nd Division, 12th Corps flag would be blue with a white star. The flag would have a red 4 1/2 inch red border on all three sides.

It would look like this flag:

http://localhistory.morrisville.edu/sites/cwrt/3dflag.html