Personally, I'd stick to a sz 7 or 8, esp if using a worsted weight. 13s will be HUGE and make the fabric very loose and swimmy (able to poke fingers thru). Whatever fiber blend you choose, make sure it's able to be machine washed and dried if for baby use. You might want to go for cotton blend. If you've not already, wander over to knittersreview.com and check out the Yarn forum. Lots of advice and suggestions. If you locate a fiber content in a more expensive yarn, and want to sub something cheaper, try knitting-warehouse.com or elann.com.