I write from continental Europe, and know very well Birkenstock, several guys said are too big...
In the past Birkenstock have 3 size:
And now have only 2 size:
Narrow is NOT Narrow, in fact it's like Regular, it's for ALL, here in my country the most common is narrow, several women style issued only in narrow.
Regular is NOT Regular, in fact it's Large, it's simple... Please choose Regular ONLY if you have very, very, large and robust/fat feet, or (the only exception) if you would to buy thong stile sandal like Gizeh, Birkenstock catalogue in my language describe the reason, thong style expand big toe during walking, and need Regular size, only women with very, very, narrow and slim feet who buy thong style sandal like Gizeh need the narrow size.
The only problem is, in some country the only size available is Regular.
I'm man i wear Birkenstock Madrid Narrow size 42, in the past i buy Regular 42 it's giant over 2 or 3 size up for me, on the big toe the space is over 5 cm, i have Regular 42 only for Gizeh, and the size is correct.
And also remember, for the correct size feet on Birkenstock need space, you have 0,5 cm on the front (big toe), and 0,5 cm on the back (heel), the reason is simple this avoid the feet ruin the border.
For the guys who said i have pain and blisters, Birkenstock are high quality shoes and foot need some days of training.
On official Birkenstock web site (.de) you found complete PDF catalogue in English/German/Netherlands.