hi bethe

3.25x5.5 is post card format.
there were plates that size, and later roll film ( 122 )
was sold that format. the cameras like the premo 3A, autographic 3A, and graflex 3A
all took that size photograph ... its a fun format


i started to type something about the other sizes / plate sizes but realize this website
says it better than i ever could ..

http://www.edinphoto.org.uk/1_early/...hy_-_sizes.htm

have fun !
john