The Linerz take up roughly 0.5 to 1cm inside the Rain Bootz. We suggest buying your Rain Bootz and Linerz one size up if you plan on putting Linerz inside.
The Linerz themselves are designed to fit the same size Rain Bootz. ie. size 8T Rain Bootz Linerz will fit size 8T Rain Bootz. Please keep in mind the Linerz are meant to fit snuggly so that they don't cause bunching inside the Bootz.