They are slightly rectangular, they conform to standard pallet sizing.
Found this on a website: Height: 1160mm Length: 1200mm Width: 1000mm Weight: 58kg Approx Includes Tank, Steel Pallet
Any IBC you get should be thoroughly flushed out if its history is unknown, go for food transport if you can (eg.molasses) as this will just require a quick clean before using in your garden. TBH I wouldn’t get one if I didn’t know its history as it could have been used for anything (we get hydrazine delivered in IBC’s, lovely stuff that).