I've rented twice and will be doing it again for my cruise in May. Most tux rental shops will measure you for a nominal fee...$5.00 sometimes less.
Go online or call the cruiseline gift services number with your measurements and a credit card and your good to go.
No fuss no muss and more room in the suitcase. Tux two shirts and even shoes if you want. When I rented on Navigator of the Seas the tux as hanging in my closet the first night... on Carnival Destiny I had to pick it up a the formalwear shop.
It's definitely a service worth paying for.
Next tux rental May 2nd Carnival Spirit to Hawaii