I have made a function to know the database length of a field, called db_string_size
http://openacs.de/forums/message-view?message%5fid=1552144 In order to use that value on maxlength attributes, and also on -validate form params.
But I thought it was easier than what I now know, so maybe I should think a little more about that. Maybe, the best solution could be "always count utf strings as bytelength".
