Standard size for a king quilt is 106x94, but could vary a bit depending on the size of your blocks. I would recommend you pick a pattern to work off of since you are fairly new to this. There are many traditional patterns available for free online. There are plenty of quilting books available & typically big box stores like Joann and Hobby Lobby carry a few paper patterns -- usually McCall's. I have not made a patchwork quilt out of denim, but a good friend from college had one that was made from all her and her siblings' old jeans. It was hand-tied with red embroidery floss and was just beautiful.
Personally, I prefer to work with denim-print 100% cotton quilting fabric rather than true denim. It is a whole lot easier and gives me a close enough look for my applique. That said, I have worked with denim before & the most important tip I would share is to make sure you buy PLENTY of denim needles for your sewing machine for the piecing work. You want to match the needle size to your thread weight. I use Microtex needles for quilting with a bit of denim (I use denim sparingly), but find I need to change out my needle much more often.