I dont see any server validation applied top this page, nor do i see a jquery pattern on the header for element to restrict the number of changacters:
<input id="contentHeader" name="contentHeader" type="text" value="<?php echo((isset($_GET["invalid"])?ValidatedField("riContentPagesinsert","contentHeader"):"")); ?>" class="formTextfield_XLarge" tabindex="2" title="Please enter a value.">
It looks like the length validation has been removed from this page.
If you select the content header form element, in the property inspector will be a max length setting. you c an use that to set the maximum number of characters to be entered.