A single size block with traditional lattice and corner stones makes for very simple and straight forward construction for any t-shirt quilt. After the stabilizing and trimming of blocks it goes together just like a "regular" old quilt.
After making a few t-shirt quilts like this, I started to branch out a bit.
Some shirts have smaller designs. You could trim these out to a larger size, but that leaves a lot of empty space in the quilt and it ends up being really big if you are including lots of shirts. If you have enough of these shirts for an entire row, you could easily trim them to 10 by 14 or even 8 by 14. Just look at all the shirts for a given row and choose the largest size that you need to cut to include all the designs.
Sometimes shirts have small skinny designs that don't take up much space. These can be combined to make a single block. Sewing stabilized shirts together isn't hard. Do pin to avoid shifting, and if possible, leave them a little bit big to start. Sew the blocks together with a standard seam allowance and then press the seams open on the back. This will help to reduce bulk as the stabilized shirts are a bit heavier than regular quilting fabric. Don't forget to use a pressing cloth so that you don't melt the stabilizer. After you have sewn and pressed, then square up to your desired size as if were a regular block.
This is my most recent quilt. The top row is 14 by 16. Some of the blocks are pieced, but some are just larger designs. The second row is 14 by 8, it has a lot of smaller designs. Notice that though each row has a different height, the widths are always the same to allow them to fit together. Using a design wall or laying everything out on the floor makes it easier to keep track of where the blocks go. Lay all of the blocks out and then add in lattice strips and cornerstones.
After sewing the top together, just layer and quilt it as if it were any quilt. T-shirt quilts do tend to be pretty heavy due to all of the stabilizer, so keep that in mind when you choose your quilting pattern. I used to tie these before I was comfortable with quilting. Just make sure that you read the packaging on your batting and tie or quilt it at the recommended distance.