Cloudflare's DDoS protection is effective for any attack size and any type of attack (Layer3/4, Layer 7 or DNS). We have no attack cap size and we do not charge overage fees. Common attack size range from 10Gbps to 65Gbps. However, we have seen larger attacks. For example, we had a customer that received an attack that was 500Gbps. This is highly uncommon and was reported as the largest DDoS in history - Cloudflare mitigated the attack for our customer. You can read about various case studies here:
The Cloudflare Network is designed to mitigate DDoS attacks in a variety of ways. You can see a full detailed explanation here.