I just finished 3 memory quilts made with pants and shirts. Much depends on how big of quilt you are wanting--usually a large lap quilt (or wall hanging) is best. and also the number of shirts available. (and the sizes! and whether long or short sleeves) these were plaid shirts, so a plainer design was used--one was a snowball (with the pants as corners) one was a railfence (with alternating plain blocks) and the other was just a simple 4 patch with alternating plain blocks. 'borders were piano keys, the half square triangles cut from the snowballs.
All 3 were 60x60 andn it took 18 shirts and 8 pairs of pants to make them.
When you disassemble the shirts, just cut off the collars, cuffs (may save the cuffs if you think you'll need that fabric) and button plackets. I used the shirt pockets "as is" per client request. Good luck!