Birkenstock use premium hides to produce thick and strong leather with a waxed finish for added water resistance, where stretching and use over time will create an interesting aged patina.
Apply a suitable water and stain protector avoiding wax based treatments on suede as this will spoil the special finish. For cleaning regularly, gently wipe smooth leather using a soft dry cloth and brush suede moving along the grain to remove dust and dirt and a gum eraser for any scuffs.
The cork footbeds come with a light coating of sealant to prevent them from drying out. When the cork no longer looks shiny apply a thin coat of cork sealant to prolong their life. For regular cleaning wipe with a dampened cloth or soft bristle brush. Allow to air-dry away from direct heat. Wearing these on most days will retain a lot of moisture so remove regularly for airing.
Birkenstocks are designed to be roomy this is needed to perform the natural rolling motion of the foot without restrictions - just as if you were barefoot.
Your toes and heel should not touch the rim of your shoes while standing in them. Most people are fine with a regular fitting, but if your foot moves side to side when you walk and adjusting any straps does not help, a narrow fitting may be more suitable.