NewLeaf is shipped with a table defining most of the standard paper sizes recognised by Windows. You can make permanent additions to this table, but please retain a copy of the code you use, as you will need to re-apply these changes every time you install a new version of NewLeaf.

nlDefinePaper 'Tall' (200 1100)

The second argument gives the (x,y) size of the paper in points. Use the same function with the paper-name only to un-define an existing paper. Save the workspace to make your changes permanent.