I think you could make it bigger. Just make sure you increase your proportions roughly the same! In fact, when I was knitting mine? I mis-read the pattern, and I did not go up 64 rows, I went up more like 90 - but I am sure its going going to be fine, it looks fine now (although not felted).
I need to get back on that I-cord. I got side tracked onto three scarves on the needles.........EEK!