I had posted this over at twc when the same question came up there. Its what CA had to say about command stars (although Foot disagrees)

https://forums.totalwar.org/vb/showp...1&postcount=15

It currently affects both morale and combat ability - we tried it for a while with just morale, but it ended up being not enough of a bonus. The combat calculations have changed so much from Rome to Medieval as to be unrecogniseable, so it's no longer easy to equate stars to experience.

As a rule of thumb it's one point of attack per command rank, up to a maximum of 10, and this can become negative for very bad generals. This combat bonus is applied to all troops under his command on the battlefield. Experience is one point of attack and one point of defense per chevron, plus a morale bonus as well.

The general's command also controls his radius-of-effect, which is set to 30 m + 5 m * command + 2 m * influence. This is used to award morale bonusses to nearby units (in addition to the combat bonus), and when testing which units are affected it tests the distance between the actual general's position and the centre-point of the unit being considered.
So according to this the attack bonus (from command stars) is applied to the entire army. The morale bonus is applied to units within a certain radius (and it looks like it can be a pretty large radius).