A shower room can be squeezed into the smallest of space as long as it is a minimum of 2 m high and 1 m wide. Think about wether you want an enclosure. Some people prefer a cubicle or you may want to keep things as open as possible, but always considering installing a screen to stop the basin and the toilet from getting wet. If you can natural ventilation is best, but otherwise invest in an extractor, and make sure your overall plumbing is up to the job, If you have a gravity-fed system or low water pressure, it may not be sufficient to support a shower in the loft, if the system is either a mega flow system or a high pressure combi boiler then there is no problem.