I'm making myself a nice big light throw to use as a pram cover for when baby arrives. I wanted something not too heavy, as we're heading toward summer.

I found this pattern, which I think will work well:
http://www.coatsandclark.com/Crafts/Knitting/Projects/Print+Ads/LW1417+Angel+Baby.htm

However, it uses a very chunky yarn. I've bought a lovely cotton fingering weight yarn, and I'm thinking of knitting the throw in that instead.

It'll be more work, but I think it'll be lovely.

Anyone who's more experienced who can see a problem arising with this, or has a different opinion, I'd love to hear it :)

Cute choice of pattern.

I've crocheted numerous baby blankets over the years as gifts. With holes a thinner yarn is sometimes too stretchy, I often use 2 strands at once. You'll have to try it and see, hope it works out as you imagined!

I'm not big on swatching, but I've read enough to know that that's the answer to your question---knit up a swatch and see how you like the look and feel of it.

Really the only way to see how a particular yarn works with a pattern is to knit a sample. You can CO about 30 sts or so (depending on the stitch count of the pattern) and knit a few repeats to see how it looks.