T-shirt quilts are so much fun to make! They are fast becoming my specialty. Just finished piecing my 3rd consignment quilt. Definitely use a sashing as it makes it more of a quilt than a craft project. I also like to use cornerstones to add another splash of color.
For my last quilt, I had gotten so many shirts with different size designs. The smallest was only 5" square, with the largest one being 15"X20". After measuring the optimum block size for each piece, I cut out graphing paper for each block and moved them around for best placement. It really turned out nice.
For the current one I just pieces, she wanted 20 shirts used and gave me several extras. I easily got 16 blocks cut at 12.5". LOVE the new 12.5" square ruler I had recently bought. Really makes it quick and easy to fussy cut those shirts. The rest of the shirts had designs that were wider than 12.5" and were only about 4" tall. I took 8 of these shirts and cut them all 6.5" tall and however long they needed to be. After piecing together the initial 16 blocks with all of the sashing, I added a strip of these extra long shirts to the top and bottom of the quilt. So far so good!