This question was posted on a popular actors forum, and I replied to their post. Here is a repost of what was posted there:
“On sites like Actors Access and NYCasting... when leaving a note for the casting director what is the best way to do it? How long should it be?”
I have used Actors Access to cast, and on the main page of submissions the casting director will only see about the first 8-10 words of the note. From there, they can click to read the full note, but they would only do that if their attention was grabbed in the first 5 words or so.
That being said, it makes sense to avoid saying something like, "I would love to be seen for this role" (that is a given, since you are submitting) or "I am submitting for the role of ***" (which is also obvious.) I would look at what characteristics are required for the role and then comment on that. If the roles calls for someone who is "perky and funny" you might want to start with something like, "I have lots of energy and experience with physical comedy." Something that lets the CD know, in 10 words or less, that you perfectly match what they are asking for.
You can also use this space to let them know your connection with the project, since the main page only includes photos of the actors (the CD has to click to view the resume.) For example, if the submission is for "My Fair Lady" and you've done the show, you could say that in your comment.
Also, I want to mention that it doesn't look bad to NOT comment at all. I cannot speak for all CDs, but it makes no difference to me if someone leaves a note or not. But if the comment is relevant and helps me to know why the actor thinks they are right for the role, it can be very useful in making casting decisions.
I hope this is helpful to you- best of luck with your submissions!