Optional Rules, Page 47 posted:

GRAND ARMY
Players who wish to play a larger game may construct grand armies instead of standard armies. A grand army follows all the standard rules for army construction, but its total point value is 1,600 instead of 800 and it may include additional units. Additionally, battles between grand armies are conducted on a 4' x 6' battlefield. During step 5 of setup, do not draw any deployment cards, and during step 6 each player has only one opportunity to eliminate a card. Finally, during step 8, each player should deploy their units within range 2 of a 4’ edge of the battlefield, opposite their opponent.