The material size is totally driven by the frame you are putting back in the road, not the one you are removing. Turn the frame upside down and measure the inside diameter. The material size you choose must be slightly larger than this diameter. if you have a 26" inside diameter you will select a 27" material package. If you have a frame with a 29" inside diameter you will selct a 30" material package.
Product Link: https://mrmanhole.com/product/27-material-package-materials-for-approx-20-manholes/
Additional Resources: https://mrmanhole.com/resource_cat/training/