What are the dimensions of the radian 3QXT? June 07, 2021 11:05 Updated 17 inches wide, 16 inches in depth. If you’d like to find out more about the Diono Radian 3QXT visit our store: https://store.diono.ca/radian-3qxt/ Related articles Can the Radian® 3QX touch other seats? Can I replace the harness straps on my Diono Radian car seat? Comments 0 comments Article is closed for comments.