You may still be able to change the color, accessories or even model of your car, so that's not necessarily moot. I understand that's all custom made to order, so if yours hasn't gone on the assembly line yet, you can probably change it.
The light blue Leaf shirt I got at the demo in San Jose fits well, so I know L is the right size, assuming Nissan uses the same manufacturer (Alstyle Green). Sometimes L is too large around the middle, but M is usually too short in the trunk. But this time of year I use that shirt as a heavy duty undershirt due to cool/cold weather and I find that one slightly uncomfortable since it is not 100% cotton. The label says 94% cotton, 6% "organic." Isn't cotton organic? Whatever it is, I notice it doesn't breathe quite as well as all cotton. I ordered the free T shirt today anyway when my UPS package came but I may end up giving it away. I can't believe they are wasting the money for those cardboard cards. One paper leaflet could contain all the printed info and pictures. Even the paint samples could come on little patches or photo paper like what you get at the paint stores instead of those thick boards. I also find the slogans rather lame and too small to be effective as advertising. They need better marketing people.