This information, dated 1907, indicates the Lookout Artillery would have flown a standard Confederate Battle Flag 36 inches square:

The Battle Flag – The battle flag is square, having a Greek cross (Salter) of blue, edged with white, with thirteen equal white five-pointed stars, upon a red field, the whole bordered with white. There are three sizes: Infantry, 48 inches square; artillery, 36 inches square; cavalry, 30 inches square. The proportions for an infantry flag are: 48 inches by 48 inches (exclusive of the border); the blue arms of the cross, 1 1-2 inches wide; the white edging to the cross, 1-2 inch wide; the white border around the flag proper, 1 1-2 inches wide. Total outside measurement, 51 inches. The stars are five-pointed, inscribed within a circle of 6 inches diameter, and are of uniform size. There should be five eyelet holes in the hoist, next the pole. The artillery and cavalry flags are correspondingly reproduced in all proportions.