LED light doesn't really diffuse thru the water like flourescents, so cutting the board down will probably leave the edges of the tank dark. Even if you pack all of the original lights onto the smaller board, you will just make the smaller area brighter. So, the bigger the board, the better if you want even lighting. Also, the big board will leave you room to add more blue moonlight LEDs or color-changing LED's later.
EDIT: Yes, packing them all onto a smaller area will increase penetration, but decrease coverage...
EDIT: Yes, packing them all onto a smaller area will increase penetration, but decrease coverage...