I would focus on buying fat quarters and 1/2 yards. I just did a top of 5" squares and I was able to get 12 sqares from a fat quarter and 24 from a quarter yard if I didn't make any cutting errors. If you don't want a whole 20 squares and can't get a fat quarter use 1/3 yard instead of 1/2. It changes the calculations of yardage. I ended up 14 squares x 18 squares and I added a 7" border because the overhang was way too small, so I'd shoot for 17 x 20 or 340 squares which would be 15 1/2 yards or 30 fat quarters or a mix of he two. This isn't counting the border. This site is a good guide for finished sizes because full covers a range of blanket sizes.
http://www.equilters.com/library/qui...bed_sizes.html