Birkenstock products are offered in European sizes. Please reference their Size and Width conversion charts below. You can determine you Birkenstock size by adding 31 to your U.S women's size and 33 to you U.S men's size.
Your feet should be able to move freely without your toes hitting the edge of the footbed.
Before trying them on, put one hand under the toe and one under the heel, then flex the footbed a few times. This will initiate the break in period and will help your new sandals feel comfy sooner.
Open all straps and step into the footbed. Close the straps comfortably, leaving enough space to allow your feet the freedom of movement. Exhale..and...Enjoy!
For the first five or size times you wear your new footwear, limit the time to just a few hours. This allows your feet and body to gradually get used to the contoured arch support as well as the straps and buckles.