The block above used 4" squares for all the patterned blocks and 4"x4.5" blocks for the white blocks. I looked at several other tutorials for Granny Square Quilt Blocks and the consensus seems to be that adding the length on the outside blocks ensures you have enough to trim a true 1/4" seam allowance around the edges. Seemed a bit strange but I did have a generous amount to square up with so I like it.
Block above measured 15.5".
But you know me and of course I wanted them BIGGER.
The second block I made used 5" squares for the center (colored) rings and 5x5.5" blocks for the outer (white) blocks. Bonus here is that you could use precut charm squares for your center blocks! I've laid them out so you can see how to add the longer squares around the outside. Match the 5" sides and have the extra 1/2" on the outside (along the length).
Sew together in rows.
Join rows, making sure your 5x5.5" blocks are going lengthwise like the other - just center them on the colored block of the next row.
Trim, making sure to leave 1/4" outside of the points!
Voila! A truly GIANT granny ;-) This square measured 19.75" - my colored squares were leftover precut batiks and they were a little wonky but I didn't mind since this was just the test block. I know these might be a bit quicker sewing them together with the strip piecing method but truthfully I like doing them row-by-row as I think I always get more accurate blocks that way. Hope this helps you if you're looking to make a granny square quilt!