Here is an idea about "how big" you could make it. If you use the charm packs their entire size minus 1/4" seams, your finished block will be 4 1/2" each (5" square minus 1/2" for 1/4" seams on either side). If you just alternated the blocks in a pattern that pleased you, you could make a top 20 blocks wide and 23 blocks long, for example (42 x 11 packs = 462 charms; 23 charms long x 20 charms wide = 460 charms). That would be an 90" x 103" top prior to sashing; this would be a full/queen size bed quilt.
The Falling Charms pattern using charms and 2 1/2" strips (your one yard of coordinating fabric will not be enough for all of the strips required by this pattern trying to get the largest size top out of your charms) makes a block that finishes 6 1/2" square (5" square plus 2 1/2" strip minus 1/2" for the internal block seam and 1/2" for seaming blocks together). With the addition of a separate fabric for the strips you could make a ~110" x 110" king size bed quilt top 17 blocks wide and 17 blocks long (17 x 6 1/2" = ~110"). You would have 3+ charm packs left over following this plan.
Of course, if you choose a pattern that cuts and reconstructs the charms so that each ends up "contributing" less than 4 1/2" square each to your top, sizes would be proportionately reduced.