Our bathroom is 2m x 3m. I am thinking of having the bath across the short wall, directly under the window and a wall-hung WC on one of the internal walls. The bath is 1.8m long and 80cm wide, which gives 20cm to spare and, allowing for the slope of the end of the bath, I imagine there would be ample room for the soil pipe to reach the outside wall OK - but there would be a long run of soil pipe. Would this give a sufficient fall? Our present arrangement was done by an optomistic plumber who fitted the WC with a near-horizontal soil pipe and we are always having problems with flushing. We have a wooden suspended floor and there is a drain cover not far from the bathroom window. We don't want the WC suspended too high - is there a maximum recommended length of run or gradient for soil pipes? 
