Yes, the ross essay has to be 300 words or less, the space alotted to the essay can't be more than 300 words. Haven said that, writing a 300 word essay was one of the hardest things I have done...It comes down to knowing what is of utmost priority to help promote yourself before the admission process.
They know you have more experience than you could describe in 300 words, so dont feel that you need to write too much...Know that every word counts, and extract the best of the best information.
Good luck