who knew ..... I have worn before - I thought a 39 or a 40 - so I went online to their website - followedthe instructions for how to measure your feet & entered the details:
"Your size specifications exceed the allowable limit. "
is the answer I get - even if I fib & put in my DS13 size I get the same answer - does Birkenstock not like me 
In case anyone reading this thinks I have big feet I do have wide feet but can get normal other brands to fit .. I was measuring as I will not have the opportunity to try in store so wanted to get it right rather than having to pay to return wrong size ....