This filter makes several copies of the original image, in a same or reduced size, into a bigger (new) image.
Tile to New Size
Input boxes and their arrow-heads allow you to enter the dimensions for the new image. Both directions are linked by default with a chain . You can make them independent by breaking this chain. You can choose a unit else than pixel by clicking on the drop-down list button.
The new image must be bigger than the original one. Else, you will get an image sample only. Choose sizes which are multiple of original sizes if you don't want to have truncated tiles.
It's in your interest to keep this option checked to avoid modifying your original image.