Depends.
Generally speaking, it is good for a response to be half-a-page long, although it really depends on the criteria question/statement and what is being asked.
Some employers will stipulate a word limit for each response. This may range anywhere from 300 words to 1000 words, so it makes sense to adjust the length according to the requirements of the employer and what they are asking for.
The general rule of thumb when writing out a response to key criteria is to decide what kind of examples and content will be included in the response and how much needs to be written so that the question/criteria is accurately addressed in the best possible manner.
Client Centric Executive Employment Solutions is an Australian based business that assists with CV/Resume writing, addressing key selection criteria and covering letters to help you with give your best foot forward to an employer. Please feel free to visit their website: www.clientcentric.com.au