If c (the velocity of light), g (the acceleration due to gravity), and P (the atmospheric pressure) are the fundamental quantities in MKS system, then the dimensions of length will be same as that of : 

c²/g

[L] = [c^a g^b \(\rho\)^c]

[L] = [LT^-1]^a [LT^-2]^b [ML^-1T^-2]^c

On solving for a, b and c : a = 2; b = -1; c = 0

\([\frac{c^2}{g}] = [L]\)