If it was me, I would select 2 or 3 complementary fabrics (tone on tone or blender prints) and work in short strips, squares and rectangles throughout the quilt until you get the size and look you want. Most t-shirt blocks are not the same size anyway so these filler pieces are often needed to even up rows or columns. You can also put sashing around each T-shirt block which will make it bigger. Borders may help too if that is the look you want.