Good thing you have little or nothing at stake here. She's listed everywhere as either 6½ or 7. The most credible source (a body double casting call for a 2013 music video for her "Britney Jean" album) says:
"Height 5’4″, 130 lbs., 32C-27′-35′, dress size 4, shoe size 7."
Most likely, 6½ is the correct length, but she has to go to 7 in most shoes due to her wide feet.