"Hardieboard" is a fiber-reinforced cement material. ("Hardie" is actually a brand name - there are similar products from several manufacturers.)
There are different types of cement-board - made for underlayment for tile floors & walls, or for exterior use as siding or soffit. It comes in thicknesses ranging from 1/4" to 1/2", depending on intended use, in sheets from 3' x 5' to 4' x 12'. If you go with the cement-board, I would recommend using one that's sold for flooring (usually 3'x5'). Regardless of which type/size you get, as nathanhinshaw said, it will have to be evenly supported.