Originally Posted by Butters
Not sure on the box without tearing it apart, but even where there is room, the cables tend to take up that space. The YTZ10s is 6 x 3 7/16 x 3 11/16 (per Yuasa). The box could possibly tolerate more width, but the cables run beside the battery so I'm not sure I'd want too go much wider (Thicker). If I recall right, not much room to go up either. Probably some room to spare length-wise, maybe a bit over 6" would fit. Ideally, probably nothing more than about 1/8" bigger than a typical YTZ10s, maybe 1/4" to play with on the length dimension.
For a more powerful battery I think the ETX18B should fit nicely assuming you have a lttle room for the height of 4"
I measured more precisely than the specs:
5.290'" L x 2.625" W x 4.05" H
They come with small pieces of adhesive foam since it's a narrower battery, the cabling may work better.