This is a blanket that a friend requested as a baby shower gift for her cousin. The request was for a blue & green carseat blanket (about 36″). I had been learning granny squares, and really wanted to try doing an entire blanket from a granny square. It took a little trial and error, because as the square got bigger it got less and less square. But I discovered that throwing in a few rows of single and double crochets every 6-8 rows or so would help provide the structure to maintain the shape. It also looks pretty cool.
Depending on the weight of your yarn, the size of your hook, and how loose or tight you want your square to be, you can chain fewer or more times between the sets of double crochets and at the corners.
One of the more challenging aspects of doing the single & double crochet rows is going back to the granny square pattern afterwords. My method was to do the three double crochets in one stitch, then skip three stitches and repeat. Once again, the weight of your yarn will determine how many (if any) you chain in between. Sometimes the math doesn’t quite add up, so you might find yourself needing to add a few more double crochets as you approach the corners, to avoid extra large gaps. I just tried to make sure that when I did that, I could make the corners symmetrical (if I did 6 double crochets on one side of the corner, I did 6 on the other side too).