Limit number of characters in input field with PHP

I’m creating a database of people using this WordPress Plugin. On the signup form, I want to limit the number of characters the users can input in certain fields, but I don’t know how, as I’m not a programmer and have limited knowledge of PHP and Javascript.
I can’t edit the HTML directly since I’m using a plugin, so I can’t just use the maxlength HTML attribute.
The signup page is based on the following PHP template from the plugin, so I guess that’s what I need to edit, but how?

<?php
/*
 * bootstrap template for signup form
 *
 * outputs a Twitter Bootstrap-compatible form
 * http://twitter.github.com/bootstrap/index.html
 *
*/
?>

<div class="wrap <?php echo $this->wrap_class ?>" >

  <?php // this is how the html wrapper for the error messages can be customized
  $this->print_errors( '<div class="alert %1$s">%2$s</div>','<p>%s</p>' ); ?>

  <?php $this->print_form_head(); // this must be included before any fields are output ?>

    <div class="form-horizontal pdb-signup">

     <?php while ( $this->have_groups() ) : $this->the_group(); ?>

        <fieldset class="field-group field-group-<?php echo $this->group->name ?>">
                <?php $this->group->print_title( '<legend>', '</legend>' ) ?>
                <?php $this->group->print_description() ?>

        <?php while ( $this->have_fields() ) : $this->the_field(); ?>

       <?php $feedback_class = $this->field->has_error() ? 'error' : ''; ?>

       <div class="<?php $this->field->print_element_class() ?> control-group <?php echo $feedback_class ?>">

          <label class="control-label" for="<?php $this->field->print_element_id() ?>" ><?php $this->field->print_label(); // this function adds the required marker ?></label>
          <div class="controls"><?php $this->field->print_element_with_id(); ?>

                        <?php if ( $this->field->has_help_text() ) :?>
             <span class="help-block">
                <?php $this->field->print_help_text() ?>
             </span>
            <?php endif ?>

          </div>

        </div>

       <?php endwhile; // fields ?>

        </fieldset>

      <?php endwhile; // groups ?>
      <fieldset class="field-group field-group-submit">
       <div id="submit-button" class="controls">
          <?php $this->print_submit_button('btn btn-primary'); // you can specify a class for the button ?>
          <span class="pdb-retrieve-link"><?php $this->print_retrieve_link(__('Forget your private link? Click here to have it emailed to you.','participants-database')); ?></span>
        </div>
      </fieldset>
    </div>
  <?php $this->print_form_close() ?>
</div>