go as big as u can afford and fit.mine is 15*25 with 3.6walls at 10deg pitch which gives 5m in the centre(about 4.8 clearspace)if u ever want to get on the roof 10deg is good 22deg is most house roofs which can be a bit slippery at that angle.def another vote here for a toilet ,ive got a kitchen and shower as well which comes in handy.ive had to build another shed as i have a compulsive toy buying dissorder
and ran out of room