Homespun lapghan...need some help getting started....

[figaro]

I have decided that I am going to make myself a lapghan out of Homespun. The problem is I am unsure as how many stitches to cast on. I am using size 11 needles and I want the finished size to be around 3 x 4 feet. I figured that I would knit up it up in squares. I came up with that size from the folded blanket I have on my lap while knitting and then rounding up. I have knit a couple of rows and so far do not have a problem with the Homespun.

Can anyone here help me out? Thanks a bunch!