Assuming that you've got your sizing instructions defined within your conditions page's text (/includes/languages/english/html_includes/YOUR_TEMPLATE/define_conditions.php, or using your admin's Tools->Define Pages Editor and choose define_conditions.php), you could set your admin's Configuration->Regulations->Confirm Terms and Conditions During Checkout Procedure to 'true' to have the "agree to these conditions" checkbox shown on your checkout_payment page.
You'll probably need to modify the file /includes/languages/english/YOUR_TEMPLATE/checkout_payment.php to change the text associated with the selection:
define('TABLE_HEADING_CONDITIONS', '<span class="termsconditions">Terms and Conditions</span>');
define('TEXT_CONDITIONS_DESCRIPTION', '<span class="termsdescription">Please acknowledge the terms and conditions bound to this order by ticking the following box. The terms and conditions can be read <a href="' . zen_href_link(FILENAME_CONDITIONS, '', 'SSL') . '"><span class="pseudolink">here</span></a>.');
define('TEXT_CONDITIONS_CONFIRM', '<span class="termsiagree">I have read and agreed to the terms and conditions bound to this order.</span>');