How big is the opening in the seat? Toshi August 10, 2018 21:42 Updated Follow The opening in the Elongated Omigo is 11.5"L x 8"W. The Round Omigo is 10.75"L v 8"W. Download our Omigo Specification Sheet for more technical details. If you have any questions, please contact us. Related articles When will I receive my order? What are the plumbing requirements? How do you operate Omigo? What maintenance is required?