How about this?
<?php
function btn($atts, $content = null) {
extract(shortcode_atts(array('link' => '#', 'color' => 'teal' , 'size' => 'large'), $atts));
return '<a class="btn '.$size.'" href="'.$link.'" style="background:'.$color.';"><span>' . $content . '</span></a>';
}
add_shortcode('btn', 'btn');
?>