Hi there. I'd say you don't need a pattern but if you want to have an idea of how big it will turn out you should do a little square (we call them swatches) to see how many stitches you need to get a certain distance and then use some simple math to have a ball park idea of how many stitches you might want to start with.
There is another simple way to make an all knit blanket and here is a simple pattern for it. LINK This is real simple, but a little more interesting than just cast on 200 (or whatever) and knit until you feel like quitting. The idea is to start with only a few stitches and keep increasing it at the beginning of the rows so that it is forming a triangle. Then when you have a triangle big enough to make you happy you start decreasing so that it will go back to only a few stitches and make a square.
You need to learn two more skills if you want to try this. They are both simple and very important skills to learn. One is the YO (Yarn over). A yarn over is not a stitch per se, but something you do between stitches. All you have to do for the yarn over you need on this type of project is to bring the yarn to the front of the work between the needle tips (like where it normally is for purling), and keep it there as you knit the next stitch. This causes a little extra strand of yarn to be made (which on the next row you work as a new stitch) between the knit stitches.
The K2tog, means knit two together and for that you just insert the right hand needle into the second stitch from the end of the left needle (where you would to knit it) and on through the first stitch as well, then do your normal yarn around and knit the 2 stitches as though they were one.
The yarn overs will make little holes and this makes the decorative edge you see in the picture. If you want to try that idea and have any questions folks are always here to help you. Have fun.