Complex sentence: The dimensions had to be slightly larger than those of a current heron, and its appearance was relatively different due to the strange beak which was was rather short, heavy and curved upwards, different from that of any other form of heron.
Split into simpler sentences: The dimensions had to be slightly larger than those of a current heron, and its appearance was relatively different due to the strange beak. The rostrum was rather short, heavy and curved upwards, different from that of any other form of heron.