So I spread the shirts out and measured side to side and top to bottom to see if I could figure out what dimension would allow me to include the important patterns on the front without running into the sleeve and neck edges. I ultimately decided that 14 inches was optimal.
But then, how to cut them? I needed to be able to see that 14" square while I cut the t-shirt fronts so that I could center the designs. For most quitlting, the easiest option is use a cutting mat, plastic cutting guide, and a rotary cutter. But I didn't have and couldn't find a template that size, and the smaller templates that I do have are somewhat opaque. I remembered that you can buy fairly thick clear plastic sheets at the hardware store, so I went to Lowes, and, sure enough, they had them in varying sizes. None of them were exactly the size I needed, but the helpful hardware people will cut them to size for you without charge. For $12, I was able to purchase the plastic and have it cut into a 14 x 14" square. (I got to keep the left over pieces. Future projects. . . .?) Not cheap, perhaps, but certainly worth it for the time it saved me.
I cut the backs off each shirt and sent them to a thrift store that recycles fabric. Then I simply placed each front on my cutting board. I didn't have to worry about measuring or straight lines or anything other than having the fabric smooth. I centered my plastic template over the emblem on the t-shirt that I wanted to highlight in the square:
Using my rotary cutter, all I had to do was cut around the template:
Voila! I hope the rest of the process goes as smoothly. . . .