Lots of questions....ok I will give my very best to explain:
1. How big is each tile ? This depends on you and your memory. I use for a large area a square by 20x20 tiles. All I have to do is to devide the areasize by 20 and then I know how big is each tile. One hint for you, if you use 20x20=400 tiles and you save 5 informations in each tile your array is of scale[2000]...be carefull with you resources.
2. For update the influence map - updating takes time and your game will slow down. But you don't have to update this that often. I'm using a master AI, which is update every few seconds and the group AI is update every second and the unit AI is update every time. During the update you have to check every unit and its contribution to the map tiles.
3. I think it is easier to use just one skill for the units status....
you.status=status_move or you.status=status_Attack
then just use define
define status_move=1;
define status_attack=2;.....
4. For getting informations like weak spots:
use the influence-map, check all tiles with tile_status=enemy(belongs to the enemy) the one with the smallest value and f.e. shortest distance is the next attackgoal.
5. To decide what a unit should do...use a formular, f.e., a big tank is coming and a single soldier is waiting just in front of him, what to do retreat or fight? Formular: decision = enemy.hp+enemy.attack_power - my.hp - my.attack_power. So if decision is < a special value retreat else attack the enemy.
6. Pathfinding is also very important in an AI, in some cases obstacle avoidance algorithm are good enough, but sometimes A*-Algortihm or similar are very important, too. But be carfull using them very often takes lots of resources (150 entities calculating a new path at one time).

So, I hope this will help you.
If you have any question please ask again.

Bye G.S.