Casting on trouble!?

I am newish to knitting. I am used to jumbo yarn size and size 10 needles. I am starting a project with smaller yarn and size 8mm us 11 size needles, which is the size of needles the yarn calls for. I am confused on how many to cast on to make a 3" x 5" childrens blanket. Help!!!

Best thing to do is work a gauge swatch over enough sts to give you about 5 inches. That way you can determine how many sts you get per inch and whether or not you like the knit fabric or would prefer a looser or tighter fabric.
What pattern are you following? Can you give us a link?