Values are saved, but then you have to select the correct option when you print the select. Is the same when you print the input field and you add
value="<?php echo esc_attr( $title ); ?>".
You can try to change the code between <select> and </select> with this
<option value="300x250" <?php echo "300x250" == $widgetads ? "selected" : ""; ?> >300x250px</option>
<option value="336x280" <?php echo "336x280" == $widgetads ? "selected" : ""; ?>>336x280px</option>
<option value="300x600" <?php echo "300x600" == $widgetads ? "selected" : ""; ?>>300x600px</option>
<option value="320x100" <?php echo "320x100" == $widgetads ? "selected" : ""; ?>>320x100px</option>