Crochet is more art than science, and it allows for much flexibility. With many crochet projects, there are multiple approaches you can take.
The granny square is a perfect example of this. There are a zillion different possible granny square variations. This comparison focuses on some of the simplest; you can change the look of a granny square just by varying the numbers of chain stitches you use when you crochet the square.
Pictured above are 4 different versions of a granny square. These squares were all crocheted using the same yarn (Bernat Softee baby yarn in Soft Peach) and the same hook (Size G / 6 - 4.2 mm). The only differences are the numbers of chain stitches used.
Let's compare them:
Granny Square Shown at Top Left: The groups of double crochet stitches are separated by 1 chain stitch; there is 1 chain stitch in each corner.
Of all the variations pictured, this one is the smallest. Notice how rounded the corners appear.
Granny Square Shown at Top Right: The groups of double crochet stitches are separated by 1 chain stitch; there are 2 chain stitches in each corner.
This is a popular variation of the granny square. It's quite pretty, and is slightly more relaxed than the version shown at top left. The corners are a bit less round / more square in appearance.
Granny Square Shown at Lower Left: The groups of double crochet stitches are separated by 2 chain stitches; there are 2 chain stitches in each corner.
This version of the granny square is relaxed and open. If you compare it against the two squares shown above it, you can see that the holes are slightly larger. The entire square is also larger than the ones pictured above.
This is a solid-color version of the basic granny square featured in my granny square tutorial.When I first learned to crochet, this was the version of the granny square that my great-aunt taught me. My early crochet work was much too tight, and she thought this relaxed version would be a good choice for me. Since then, I've learned to "loosen up" on my crochet work, but I still enjoy working this version of the granny square.
Granny Square Shown at Lower Right: The groups of double crochet stitches are separated by 3 chain stitches; there are 3 chains in each corner.
This granny square is getting to be too relaxed; there are too many chains. You can see that there are unsightly bumps and buckles in the work; this effect is not recommended.
Note that everyone crochets a bit differently. It so happens that this is too many chains for the way I crochet. In general, I think it is probably going to be too many chains for the way most people crochet, but there could be exceptions. This design could possibly be acceptable for use by crocheters who make very tight chains in proportion to the rest of their work. Generally speaking, though, I think most crocheters will achieve better results with any of the other variations pictured.
If in doubt, feel free to try several of them to see which one works best with the way you crochet.