Sending a large parcel from Chengdu to Sri Lanka

I want to sent a large parcel which is around 20 kg from Chengdu to Sri Lanka by China post. No matter how many days it takes to reach the destination. How much will it cost? What should be the maximum length, width and height? If you have any details for length, width and height and the related weight, please let me know. Thank you so much for your concern on this.