I found a great book last year on making t-shirt quilts that really helped me.
Use featherweight fusible interfacing. Anything heavier will really make your quilt stiff. Before I start cutting any shirts I number them all with a piece of blue painter's tape. I like to use excel to set up a spreadsheet with the shirt number, color and dimensions. Always work with finished EVEN numbers. Most designs will end up being 10" or 12" wide. I hate to see a 12" block cut out of a shirt that has a 5" square design. Be sure to add your 1/4" seam allowance. Once I have all of my shirts measured I try to sort them into 3 to 5 groups based upon those having the same width. Add up all of the shirt heights in each group and add in 2" for sashing. You can always add some borders around smaller t-shirts to get them up to a usable size. This is why it's so important to work with all EVEN numbers. You can also look back at some of your shirts to see if you can reasonably cut a design an extra 2" wider or taller.
Here's the one I just finished.
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