Yes! The probability density is:

The value of y2 is a maximum when sin npx/l is a maximum. The maximum of any sine is 1, so:
y2max = 2/l
The length of the retinal p-chain, which has 12 atoms is:
(a) 11X0.15 = 1.65 nm, or
(b) 12X0.15 = 1.80 nm