I need to rehang the shower head fixture base in our '99 Contessa.  The fixture base is attached using a pair of simple anchors, one of which has worked its way loose (and the other is losing its grip as well).  I plan on replacing using the kind of anchor that expands to grab the back of the wall.  These come in various lengths, so I need a good idea of the thickness of the wall in order to get the right size anchor.  Any thoughts?
Larry