Making a blanket to cover a damaged couch cushion. I need help with starting chain length and which stitch to use.

The corner cushion on our sectional sofa has been ripped, and between kids fiddling with it and a dog trying to dig it even more, we need to cover it. I figured I could crochet a simple blanket to cover & protect it.

I ordered Bernat Blanket yarn and have an L hook to use with it.

If I am making a blanket that’s about 52’x50’, how long should my starting chain be? And which stitch would likely give me the best coverage with the fewest gaps?

I’ve never used a Bulky or Super Bulky yarn, so any help would be appreciated!