When designing a Custom Set Quilt, on the LAYOUT tab there is the option to Maintain the existing block sizes while resizing the layout.
For example, here is a quilt with the center measuring 24 x 24.
If the size of the quilt is changed to 48 x 48 with Maintain the existing block sizes while resizing the layout unchecked it looks the same because the blocks are resized along with the quilt.
If the size of the quilt is changed from 24 x4 24 to 48 x 48 with Maintain the existing block sizes while resizing the layout checked there will be extra space to work with.
If the size of the quilt is changed from 48 x 48 to 36 x 36 with Maintain the existing block sizes while resizing the layout checked the quilt center will surpass the borders.