I’ve finished up 2 security baby blankets this week. (I like having a spare). They are super easy and done entirely in half double crochet out of Homespun yarn found in Michael’s. It’s super soft and squishy and 100% acrylic.
I cast on approximately 45 stitches and crocheted with half double until it was square. Then, I half double crocheted around the whole perimeter to give it a more finished look. They came out to about 28″ square. Each blanket used a little over 2 skeins.
You could single or double crochet instead of half double and it would turn out just fine. I like the slight openness that the half double provides.