You could do away with a base altogether, then you can have any size you like - we bought a concrete lintel for €9, used an angle grinder to cut it to the size we wanted and then tiled the base of the shower and up over the lintel. Shower door on the top - job done. Only tricky bit was making sure that the floor sloped to the plughole. Hubby is a handy DIYer, took him 3 days to do the above and tile floor to ceiling in the rest of the en-suite.