There is no inherent penelty for depth of missle units like there was in MTW, so you can have very deep missle formations and still be extremely effective.
As for infantry units, while there are bonuses for depth, they aren't great enough to compensate for the bonus one gets for 'wrapping.' Wrapping is where the men of your unit wraps around the edges of the enemys formation. It can also go the other way, the enemy wrapping around your formation. As such, about the most important factor in depth is 'wide enough that your front is equal or greater than theirs.' Remember though that vs cavalry/chariots its better to have a deep formation as that tends to engulf the charging unit into had-to-hand fighting. So theres a trade off that depends more on who/what you are facing more than enything else.
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