Borderless printing on Canon printers can only be done on certain specific sizes of paper because the barrier covering the foam pads under the trace of the print head has to have openings to receive the overspray. However, you can print a bordered print on 8.5" by 14" (as fotofreek suggested) and cut it down to size.
A printer capable of A3 like the i9900 can print up to 13"x23" but it still will not allow a borderless print on 8"x12" for the same reason that the iP4300 will not.