I'm curious.. did you accually try using the deco marker before deciding to make a thread for help? Did it click in your head automatically that it would be to small? Or did you try the marker and then realize this catastrophe has stumbled upon you?
If you are expecting a real answer, my advice would be to find a screenprinting company and have them print it custom for you. It may cost a little more in the end, but it's better than having it turn out shitty and wasting a sweatshirt and time.