Hi,
How large is the afghan now? If you wanted to make it larger, you could crochet a border around the afghan for several rounds. I need the size to really suggest something, because a large border isn't going to look that great. I make granny afghans, and I put a border around them as a finishing touch. I usually only make two or three rounds and the border looks fine with being that width. I think a wide border would not look as nice. That is why I ask for the size. If I knew the size, I could have an idea of how large the afghan already is and maybe I could suggest something that would be more appropriate, but not having a size stated, the border is all that I would suggest because this is a good way to finish up granny afghans. Please post the size of the afghan here so that I can help you with better advice.