If you have an old shirt, that is too big for you still, you can take it apart, and use the pieces for a pattern to make any new shirt.
If you use a worn-out current shirt, that is tight, you can adjust the measurements to make the sleeves longer, the shoulders wider, The chest/stomach wider, etc. Take your current measurements( the help of a friend is just the ticket, here) and record them.
When working with any leather its better to make the garment too big than too snug. They are difficult enough to put on and off, when they are too big. They are next to impossible to get off if they are too snug, unless you stand in the shower and wet the clothes down.
Leather is HOT in the summer, and COLD in the winters. A oversized shirt can be worn OVER another shirt to provide better warmth in the winter time. The Book of Buckskinning has very good color photos of leather hunting shirts that can inspire you. And even the pictures in the Crazy Crow site will give you a better idea of what these shirts should look like.
Oh, last comment: If you want to actually wear this shirt Hunting, put the rough side of the hide OUT. Most people make shirts with the shiny side out. This reflects a lot of light, and makes any movement of your upper body easy to see even in the woods. If you use the rough side out, light is Not reflected easily off its surfaces, and movements are much harder to see. The darker, rough side also seems to blend in to the background in woods, than the shiny side will.
Finally, putting the slick, or shiny side inside will make it easier to slide a wet( from body sweat) leather shirt off your body, and off any undershirt. I joined two other club members, years ago, to help pull a hunting shirt off another member, where he put the rough side in, and wore the shirt in a parade, in July! It took 3 of us, together with the shirt wearer, to get the thing off. One each pulled on the sleeves, while the third lifted the shirt up and away from his skin on his back. He pulled up on the shirt in the front, when we alternately lowered one arm or the other. :surrender: :thumbsup: