There is no "Multiple" Search Option at this Time, but you could select "I don't Care".
It works for me by holding the ctrl key down to select multiple body types or relationship types etc..
I just tested again and searched only for thin. got 18 matchs... Added athletic to the thin and got 24.... added average and got 67. I checked through who was selected and they all matched my multiple criteria by using the CTRL key to select multiple options.
Could this be a new feature added? Admin sometimes slips new features in without much announcement?
I am NOT arguing with ya! But this seems to work fine for me. Give it a try.