If you want a deep plush blanket, with a 10.5 needle and #4 yarn, you'll want to knit double..
The stitches for a big blanket will be crammed onto the circular (you can comfortable get about twice the width of knitting as needle (a 30 inch needle= 60 inch blanket. (29 inch needle 58 inch blanket.
me? for simple plush warmth i would go with a MOSTLY garter stitch blanket
work with 2 yarns held together (you'll be getting about 2.5 to 3 stitches per inch.. lets say 3 stitches per inch.. 56 inches width.. about 160 stitches
as for the pattern?
R1: K10, *slip 1, K10, (repeat across row)
R2: K5, *slip 1, K10, (end row with knit 5 )
this is not a hard pattern. Its an easy one to see (so you'll be able after a few rows know exactly what to do!)
it reversible (but not exactly identical!)
If you want a wider (or narrower) blanket, the pattern works with any 10 count (100, 110, 120...160, 170, 180!)
the garter makes a nice deep ridge that make the blanket warm.