Hi-
Think about it this way- if your boss is addressing a "weakness question" that says "what was an important feedback that you gave to the applicant etc. etc. and how did he respond?" ( 500 words)
A recommender should easily take up more than 350 words for this( or may be touch 500). Why? Well, in the first half of the response, he will set the background for a layman( READER) who has no idea about the relationship between you and your boss. And in the second half, your boss will show how you took corrective actions, and how has he seen you improving on that weakness in the recent past.
The reader of this recommendation will not make any phone calls to you or your boss, and will NOT have any other means to complete the assessment other than that response. B-School applications are like- ONE Sided communications where the rule is- If I am not able to see the impact of an essay or a recommendation, I am not allowed to call up the writer and ask for MORE!
In this scenario, I will suggest that you use whatever space is given to you.