i made a game a few years ago and am now remaking it in higher quality in photoshop.
the board size is 26 inches by 26 inches.
my question is how should i make a backing for it.
i have my ideas but was wondering if somebody else has better ones.
the board art is the 26 inches squared. should i break that into 4 13 inch squares?
I'm not sure exactly what your question is. Are you looking for pointers for the backing material to use, the board material itself, or how to create the artwork?
If you're looking for tips on how to make the board iself (the actual board core and tips for applying a backing) see this thread.
For more information on the actual types of board used as the core of game boards, check out this thread containing a game maker's glossary.
Your question about breaking the board into 13" squares makes me thing that you're asking about the cor of the game board itself (not the face artwork or the backing material). If so, then your best bet would be 2-ply chipboard (also called boxboard). You can make it according to the instructions linked above or as a quadfold as you like.