nothing too long; nothing that has an offensive, discourteous or bullying tone;
not too much from one particular commentator (ie imaximum of one or two Comments on any one topic from any individual);
nothing anonymous unless the person concerned sends a separate email giving his/her name, and possibly not even then if the comment is too rambling or discursive or rude; and
nothing that simply prolongs a discussion once I feel it is time to draw things to a close.
These are roughly the rules used by newspapers for publication on a Letters Page. Newspapers also reserve the right to edit or shorten letters, but I am unable to do that on a Blog.