A Birkenstock size is unlikely to correspond, or have much value in judging size in other brands.
Most companies that size their products in the European sizing system come fairly close to what might be considered a universal size translation chart. OMC does not sell Birkenstock, but upon looking at Birkenstock's own official sizing chart, their translation is quite far off from what is usually agreed on. For instance, most sizing translations put a US mens 11 at a 44.5 or a 45. Birkenstock's translate their size 45 as a US 12 to 12.5.
Best advice is to have the foot measured on a Brannock devise that uses the European scale.