It is not clear where to indicate size. Are you actually purchasing a gift certificate?
Mindy asked on Jan 20, 2013
All redemption instructions are included on the Groupon once it is purchased and printed.
You can find and print your Groupons by logging into your account at www.groupon.com. Once you are signed in, place your cursor over your name at the top right of the screen and click "My Groupons" in the drop-down menu that appears.
After you’ve reached that page, click “View Voucher” to open your Groupon in a new tab. To print, just click "Print Voucher".
Joseph G. from Groupon replied on Jan 22, 2013