A general battle formation I take when the field is large enough would probably be your standard legionary center with ranged support at various points along the line with a few infantry reserves. Something like this for most large engagements:
KLLLLK
SK JLLJ KS CC
K=Skirmisher
L=Legion
S=Slinger (or any other ranged support)
J=Javelins
C=Cavalry
I like this formation because it presents a strong force of legionaries up front for charging or bracing, and skirmishers on the flanks to go for any flanking ranged units. In the second row javelins and archers can shoot over top of your center troops, as well as some ranged support on the flanks you can move to any opportune spots to fire on engaged enemies, and a back up unit or two of Legionaires and skirmishers to provide support to depleted units or form an emergency line against flanking attacks. Finally the Cavalry units can be positioned on either flank to either protect weak spots in your own line or punch through weak spots or ranged support units of the enemy line.
EDIT: My letter diagram is a little off center because of the posting format, but I think you'll get the point.
