Check despair.com in the headline for some wonderful creative work.
Previously I wrote that Bigger was usually Better in media. One exception to this might be posters targeting young people. Levis had some research showing that young people saw large posters as too corporate so they used smaller bus shelters
And remember that creative on posters starts to wear out after only 10 days so change your ads frequently. Media owners don't give discounts for using worn-out creative work so it's better to change copy too early than too late
Think of a calendar. After one month the image is dead.
Another exception is mobile outdoor. Taxis & Buses etc. As you never know when buses appear the surprise gives the creative work a longer lease of life
Creative is probably more important with posters than any other media. Bad TV ads still work - think of Shake & Vac but bad posters simply aren't seen - if in doubt remember that Sex always Sells