Unlike STW and MTW, I don't think we know quantitative information on the RTW/M2TW engine unless it's the kind of stuff you can get at in the txt files. CA revealed a lot of the maths of STW/MTW, not least in a chapter in the MTW strategy guide. However, with RTW, they explicitly refused to give such information citing intellectual property rights and fear of copying by rival companies producing "clones".

On command stars in RTW, a CA staffer posted here that they were intended just to raise morale but then made to affect attack stats because the morale effect was too weak. However, therother reported that his research revealed no noticeable affect of command stars on combat kill rates, so the current consensus is that - contrary to the CA statement - the game actually reflects the original intention and command stars only affect morale. This is discussed here:

https://forums.totalwar.org/vb/showthread.php?t=44549

Exactly how big the morale effect is and whether it depends on proximity to the general is unknown. It should be easy to test if anyone had the energy to do it (mod the descr_strat and EDU to match up a unit under high command with a comparable unit with high morale, and record when they break).

My guess would be 1 command star = +1 morale (but perhaps only if near the general).