Always show same size tags for Tag Cloud in WordPress Admin

use this at functions.php

    /* Tagcloud, change the font size */
function custom_tag_cloud_widget($args) {
    $args['largest'] = 13; //largest tag
    $args['smallest'] = 13; //smallest tag
    $args['unit'] = 'px'; //tag font unit
    return $args;
}
add_filter( 'widget_tag_cloud_args', 'custom_tag_cloud_widget' );