AI behavior. While you cant directly edit the AI behaviors they are slightly edittable. Basically there is a list of set hardcoded behaviors that you can choose from to apply to a faction. Im not sure exactly where this is because ive never done it before but i know it has been done.

Changing costs. You can change unit costs in 'export_descr_unit.txt'. The same can be done for buildings but im not sure of the file. Along with changing building costs you can give other attributes like law or happiness or squalor bonuses.

Yes you can change unit sizes. This is done easily. From your start menu go into options>game options (or maybe it was video options). Anyway. Here youll be able to set the resolutions. Beneath that is a check box asking to display advanced options. Check that it should be one of the menus. Each time you increase the unit sizes are doubled. (maximum unit size of 240). The highest level can be a bit taxing on your computers capabilities though in the bigger battles. Also these large unit sizes can quickly deplete the populations of your cities especially early in the game.

A great place to learn about modding rtw is to check out the scriptorium. This forum contains tutorials to do pretty anything you would want to do as well as links to modding tools. It can be a long read but once you get the hang of things it really isnt too difficult.
https://forums.totalwar.org/vb/forumdisplay.php?f=77