Hey Heather, just my two cents as a seller who does a high volume of custom work and most of my ready made items come in different sizes.... I do get customers who do not read the listing info and don't include the customization information when they check out.
I NEVER guess, I NEVER just ship. Ever. I try to contact the customer in as many methods as I can. If I still have not heard from them by the shipping date, I send one more message to say that I need the information in order to process the order. If I still don't hear back, I refund the money and cancel the order.
You can't get a bad review on a cancelled order. A customer who has been contacted repeatedly and not responded can't leave you bad feedback for doing your due diligence. They made a mistake, you are trying to remedy it. You will get bad reviews if you just ship something and hope it is ok - what if it is not? Then you have to spend more money to remedy that situation. Better to get it right the first time, and in my experience, customers appreciate you trying your best to get it right.
Honestly, it has been less than a day. Give the guy a chance to get back to you. It is a crazy busy time of year so people might not check and respond to email immediately. Don't just send what you think he wants. Tell him if he wants it by Christmas you need to send it today. If you don't get the info today, give him the option to expedite it. He may also choose to just cancel the order, which might seem like the end of the world when you are new but it is better than an unhappy customer and a bad review.
Good luck, and happy holidays.