To figure out how many stitches you need, you have to know how wide you want it, and how many sts per inch you get with the yarn and needles you use. Multiply the sts/inch times the inches and that will give you a cast on number.
You wouldn't want to use even that long a straight needle for a blanket, the weight would be too heavy after an inch or so besides not being able to fit them on. Circular needles can be used to knit flat items, that's how you fit them all on.
sue- knitting heretic