Hi Becky,
I found adding 200-300 words for the category pages worked best for my ecom site. I use 400-500 words for the main body on the products I sell. I (somehow) ranked number one on Google the previous 2 years based on organic content writing - no backlinks
My ecom site is LikeChimp.com - you can see how I use good content to rank, check out the page categories vs the product description: http://likechimp.com/product-category/buy-facebook-event-attendees/#
Hope this helps.