3 1/2" sashing strips would get you up to 12 1/2"---but that large could over power the block
visit Quilter's Cache---they have tons of free blocks in alot of different sizes- you just choose what size you want then find the block- you can make a block 6 1/2",i think 8 1/2, 9", 10 1/2", 12 1/2", 15"...they have them all- and it is a good go-to site for free blocks.