Birkenstock use a robust and breathable textile.
Apply water and stain protector before wearing and after each clean. For regular cleaning allow any mud to dry before gently brushing off. Refer to manufacturer’s care instructions for a machine washable option. Many stains can be removed with a sponge dampened in cool water using a neutral cleaning agent not containing chlorine or bleach which can discolour and weaken the fibres. When removing stains by hand apply gentle pressure to avoid leaving friction marks. Always rinse. 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.