Often times if a background is non-specific
(landscapes and skies, not inside usually) than
the background will simply be painted and added,
there is no need to paint a sky a certain size
because it does not effect the character. For
complex scenes in which the character may interact
with the background, the background will usually
not be oversized UNLESS it is a pan or zoom in
scene. If you email me a picture I might be able
to be of more assistance.

So long,
    Larg0