Max carry on size for cruise

[Wendysofftoneverland]

Was planning on having my roller suitcase that fits Southwest carry-on size, but it looks like it might be one inch wider than the Disney Cruise Line carry-on size. How strict are they? Everything I have read seems to indicate the sizing is based off the bags being able to fit through airport scanners, which mine is actually smaller in 2 dimensions than the max size for Southwest.