Macs Auto Parts list the battery size for the 57 and 58 Fords as 29N and O'Reilly Auto Parts show that they have a 29NF battery that is stated to fit the 57 Skyliner. I have a smaller battery in my 57 Skyliner (Super Start 35EXT that is 9.6" long compared to the 13" long for the 29NF) that fits the original compartment (with room to spare, of course). The 35EXT has better performance than the original size listed by O'Reilly but if you are looking for the original size, the 29NF is listed for that car.

Good Luck,
Dave