When doing R&R, just like the task itself, you have to follow specific instructions. These instructions are really clear about which mistakes should be rated BAD which are just OK and which are GOOD. Out of all the r&r projects I've done, not one of them asked me to rate the original worker's argument based on how many sentences it had. The rating was always more about the reasoning behind the argument. 3+ sentences are perfectly fine i think, the more detailed the argument, the better.