When I was playing my first campaign (as the Scipii romans) I ended up building "standard" army stacks that I almost always deployed in the same formation for field battles. I usually had the following units, with some variation to stacks (more cavalry and less artillery for a more mobile stack, replacing 1-2 archers with special units like merc elephants, etc)

1 general [G]
1 wardogs [D]
4 cohorts of "elite" infantry (eg Praetorians) [E]
4 cohorts of "normal" infantry (eg Legionaries) [N]
2 cavalry [C]
4 artillery (onagers) [O]
4 archers [A]

[C][ E ][ E ][ E ][ E ][C]
....[ N ][ N ][ N ][ N ]
[..A..][..A..][..A..][..A..]
.....[O][O][G][O][O]
..............[D]

This suited my style, which is to attack the enemy in the strategy map, move my army to artillery range and bombard them untill they'll either attack me or run away. I don't like using infantry offensively (except in siege battles), I always just make them form a line and put them in guard mode and let the enemy come and fight. The only time I ever move my infantry is when moving the whole formation to take better ground right after deployment, and to move closer to allow my onagers to fire. Their job is to be an inpenetrable wall of steel, while my cavalry and archers do most of the manouvering. It's also easier to maintain control over things when I don't have to worry about micromanaging my infantry much.

Although romans don't use phalanxes, their infantry is still very good for this sort of tactics. Especially with the shower of javelins just before melee contact, which can often rout charging infantry units before they even clash with my line (due to being already weakened by my archers and onagers). Deploying in two lines of cohorts rather than just one long line makes a shorter front but I've found 4 cohorts to be long enough for most battles. The 2nd line is handy as they can step in as fresh reinforcements when my 1st line is beginning to lose. The reason I put my elite in front and normal infantry behind them is that the elites can better withstand the first clash when the enemy charges. By the time I have the 2nd line step forth (if it even comes to that) the enemy units will already be reduced and exhausted so the quality of troops will not matter as much as being fresh and at full strength.