Well every time it gets rid of a shell it grows 1 1/3 times larger. So so to see how much it grows after the first time you say 1 cm * 1 1/3 = 1 1/3 cm
to get the next one you do the same 1 1/3 cm * 1 1/3 cm = 16/9
It will keep going multiplying by 1 1/3. So so we can say in an equation that the (initial size) * 1 1/3 *(number of shells) = length