The 48" x 48" max size is a reasonable max size in Pcbnew to avoid overflow in calculations,
due to the internal units set to 1 nanometer.
Gerbview, Eeschema and page layout editor, having bigger internal units, can manage bigger page sizes.
Because a page having a size of 120" will be not easy to print or edit, perhaps you should consider using a (flat?) hierarchy instead of only one large page.
The 48" x 48" max size is a reasonable max size in Pcbnew to avoid overflow in calculations,
due to the internal units set to 1 nanometer.
Gerbview, Eeschema and page layout editor, having bigger internal units, can manage bigger page sizes.
Because a page having a size of 120" will be not easy to print or edit, perhaps you should consider using a (flat?) hierarchy instead of only one large page.