I have found that the easiest way to do the kind of layout you linked to is to set a common denominator for your t-shirt blocks. I usually go with 3 as my common denominator. Then you can cut, based on imprint sizes, any size that is evenly divisible by 3. which would give you 3", 6", 9", 12", 15" etc. Then choose a common size for your sashing or let your sashing be your "fill in" for getting odd sizes to fit. Get out the graph paper and start sketching. On this T-shirt quilt linked here:
Double sided T-Shirt quilt I started out that way but some of the imprints didn't play nicely with that size so I made it a shadow box layout and used my sashing (the light purple) as a background color and used it to fill in the gaps. The quilt you linked to appeared to me to have partial seams. I could easily pick out the blocks of imprint except the right side edge and top right edge appear to me to be constructed with partial seams.
Edited to add don't forget to add your seam allowances once you have decided on sizes.