The size of the battlefield depends on the size of the game played. A standard game of 2500 points is best played on a 72" x 48" table.
Firstly, place the terrain you want to use. Rules are not strict here, but a good guideline is placing one terrain piece for every 24" x 24" square of the table.
Players take their turn to place the terrain pieces on the tabletop. Once finished, both players should agree on what sort of terrain each piece is to clarify what certain rules they might have during the battle.