Birkenstock use premium hides to produce thick and strong leather that is buffed on the inside to create the supple suede softness.
Apply a water and stain protector before wearing and after each clean, avoiding wax based treatments on suede/sheepskin as this will spoil the special finish. For cleaning regularly use a suede brush moving along the grain to remove dust and dirt and a gum eraser on suede for any scuffs. Sheepskin rarely needs washing as airing is usually sufficient with moisture vapour passing through removing any impurities. Although, if necessary, dampen in cool water with a mild detergent, then rinse and allow to air-dry naturally away from direct heat.
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.