Well, if you have 3" strips and they finish at 2.5 and 2" strips and they finish at 1.5, you have 1.5 + 1.5+ 2.5 = 5.5, so if you cut your focus fabric at 6" square you'd be alright, although your quilt would be smaller.
Personally, I use cut an 8" focus fabric (7.5" finished) and then cut 2.5, 2.5, and 4 to get 2 + 2+ 3.5 and a finished 7.5" block. Eight blocks across is 60" and ten down is 75" or throw on one more row for 82.5".
I find it gets done faster with bigger blocks, but that's just my way
