Not had the chance to test this, so try it at your own risk...
In the tratings_feedback template, find
PHP Code:
<input class="bginput" type="text" name="usercomments" size="45" maxlength="80" value="$trating[usercomments]"><br />
Change the maxlength parameter to the maximum you're after.
Then edit the user_rate_trade table... the usercomments field is a varchar with the same maximum length. It could feasibly be a lot larger than maxlength parameter for the input anyway, but as it's limited by the input there's not much point.
I'm at my office PC, so I've not been able to check/test the above, but after downloading the script it would seem to be all that affects the comments - being limited by the maxlength in the input, there's no need for any validation of the length, so this should work.