Ideal length of a dipole for SWL

Hi, I want to setup a dipole antenna on my terrace for SWL (mostly for HAM bands and air traffic).  What length should be ideal to cover both 20 and 40 meter bands ??  I need to keep the length minimum as I live in a rented apartment and the total area of the terrace is around 12 meters X 12 meters with no trees or a y other support around. Also, do i need to further put this dipole higher from the terrace?? or can I put close to the wall?? (Actually I dont want pipes or fixtures on the terrace, want to keep it minimum..) Please suggest any other ideas if you have.