The absolute minimum size for a walk in shower comprising of a single or double shower screen setup would measure 1000 x 700mm. At 1000mm wide this would incorporate a small 600mm shower screen with a 400mm entry space. The other end would either be the wall in a recess setup or the other shower screen at 600 or 700mm in length if installed in a corner. These are absolute minimums. A more comfortable walk in shower would measure 1200 x 700mm and upwards. If you only have this much space consider a shower door instead.