A square boiler? Most have one circular dimension, making a cylinder.
But as your post suggests it isn't a cylinder, first thing to do is . . .
. . . - calculate the cubic dimension. As your measurements are in inches, we'll convert to cubic inches, then convert to liters.
Multiply the three dimensions together to obtain cubic inches: 24" * 13" * 13" = 4,056 cubic inches
Second, go to:
http://www.convert-me.com/en/convert/volume, scroll down to cubic inches and enter 4,056 in the box. Scroll up to liters (or any other conversion you want) and read the volume.
George