Can you please tell me the maximum height that a child can be to use the harness?

Hi Nika, thank you for your inquiry, The height l.imits for the Diono 3RXT 2020 is: Rear facing: 112 cm or less. Forward facing: 144 cm or less. Booster: 144 cm or less. We hope this helps. if you follow the link https://www.babyonthemove.co.nz/store-locations/ this will take you to our online store finder. Here you can find contact details for your nearest Baby On The Move Store where one of our team experts will be happy to assist you in your product search and help answer any further questions you may have.