the best way to find out is to make a little "swatch".

Take the hook and the type of yarn that you want to use for your blanket, then chain 25-30 , then continue on and crochet a little square in the stitch that you'll want to use for the blanket. After you've got your square, measure how many stitches you've got per inch in width, and then how many stitches you have vertically as well.

then you will measure your bed and decide how many inches wide and tall you'll want the blanket to be and you'll use the numbers from your swatch to figure it out.

For example, if in your swatch you had 3 stitches per inch wide and 2 stitches per inch tall and you want your blanket to be 45 inches wide and 60 inches tall: 45 inches x 3 (stitches per inch) = 135 stitches (how many you'll chain and how many stitches wide the blanket will be) 60 inches x 2 (stitches per inch)= 120 rows = how many rows you'll crochet on the blanket.

But, the height of the blanket is much easier to do than the width, because after you'e started it you can just keep measuring it or placing it on the bed to see if it's the length you want.

here's a link

http://ezinearticles.com/?Gauge-Crochet&id=85870 about swatches.