I searched for tutorials on how to make it bigger, and found complete uniformity in technique: cut the shirt up the sides and stick in a panel of fabric. Wow. And how many people are going to recognize this as 'you made that shirt bigger because it was too small to fit you!' I wonder. Also, it seemed since I was making it so much bigger, it would distort the garment so much that I probably couldn't wear it (can you say, huge sleeves?). So, here's what I did.
Measure your original shirt dimensions at several points, length, width at bust and bottom measures, or as many places as you deem needful, etc.
Laid the shirt out flat, front to back, trying to keep the fold between front and back along the underarm/side seam. Started my cuts roughly halfway between the shoulder seam and neckline, and cut the shirt into rough thirds, trying to keep the outer two pieces the same size as each other. Each cut was angled inward toward the middle, so the line would make a V-shape, and give a slimming impression, I hoped. You might decide to make your shirt-dissecting cuts parallel with the side seams of the shirt, which is a perfectly good choice, too, if you like that better. It reminds me of a bowling shirt, a bit, which was another factor in my decision to make my cuts slightly diagonal.
Then, I measured a T-shirt that fit me, and determined the amount I'd have to insert (subtracted the original measure of the shirt I'd just cut apart), divided that by 4, and added seam allowances. Cut my color-matched fabric (woven, even though the shirt was a knit) by that measure. Since the edge of my fabric was cut with a slight diagonal, I just went along that, measuring every few inches to maintain the proper width of the fabric piece. Since it was going to be in a slight diagonal orientation, anyway, plus the slight bias giving the woven fabric a bit of stretch, I figured that would be fine. If you're using a contrasting fabric or distinctive print to patch in, you may need to figure out if bias or diagonal works for you, and place your cuts the way you think they look the best along the print, etc.Here is the finished garment:
