Inside length of the shoe
match to an inside shoe measurement that fits you well 
size 11   = 16.5cm
size 13.5 = 18.5cm
size 1.5  = 19.5cm
size 2    = 20  cm
size 2.5  = 20.5cm
size 3    = 21  cm
size 3.5  = 21.3cm
size 4    = 21.6cm
size 4.5  = 22  cm
size 5    = 22.5cm
size 5.5  = 23  cm
size 6.5  = 24  cm
size 7    = 24.5cm
size 10   = 27  cm