Does this only affect what is trained? Can they grow through exp and combat by themselves or is it hard capped by this stat/ability?
Not sure I understand the question, but if it means what I think you're asking, then no, units that were trained with X figures will always have X or fewer figures, and you cannot upgrade that to Y figures in any way currently in the game. I believe that upgrading unit sizes can be modded into the game, but I don't know how to do that.
It has no effect on the experience gain, but it does affect the maxium health - calculated as
"(BASE_HEALTH + UNIT_LEVEL*LEVEL_HEALTH_BONUS)*SIZE_OF_UNIT"
where BASE_HEALTH is six for trained units, UNIT_LEVEL is what you would expect it to be, LEVEL_HEALTH_BONUS is three for Ironeers (Gilden), one for Amarians (New Pariden) and Wraiths (Resoln), and two for everyone else, and SIZE_OF_UNIT is the maximum number of figures in the unit (3, 4, 5, or 7).
Note that the damage increase from having larger groups isn't exactly SIZE_OF_UNIT*UNIT_ATTACK; rather, you get CURRENT_SIZE_OF_UNIT attack rolls, which means that the damage your unit does is determined by how many of the figures in your unit successfully hit the target and did not have their damage blocked by armor. If you have seven figures in a unit, then that unit can have anywhere from zero to seven successful hits on a single attack, with the attack damage determined separately for each hit and summed to give the damage amount that shows up on screen.
