Character limit on body Summary field
The answers on this question might help you.

You override maxlength in a hook_form_alter(). 

function hook_form_[content-type]_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    $form['summary']['#maxlength'] = 40;
}


The above is not tested but should get you started. You can inspect the $form object with dpm() if you have Devel installed to check the exact name and place in the $form array of the summary field.