There is no upper limit, bigger means more attractive.
Minimum effective value is 6 though... 5 or less will not trigger anything.
Some notes:
- aliens will not pick up a weapon, if they already have a working weapon
- aliens will NOT pick up weapons you throw them... they consider them traps... so only weapons they have dropped, or you have lost when dying during their turn
- aliens will NOT pick up weapons if they currently see any xcom operatives.... don't ask me why
- aliens will NOT pick up a weapon, which they don't have ammo for (except blaster launcher or other waypoint-based weapon)
Example:
a/ first a survey is made for the most worthy weapon from all weapons on the ground (which are not considered traps)
Worthiness = attraction / (distance(unit, item) * 2 + 1);
e.g. for attraction = 10, distance between unit and item = 2 tiles... the worthiness = 10 / 3 * 2 + 1 = 10 / 7 = 1
b/ second, once the most worthy weapon is picked, aliens check if they have ammo for it and if they dont see anyone. After that they will make a move for it if the following formula returns true:
attraction - distance(unit, item) * 2 > 5
e.g. for attraction 5 or less, this always returns false
e.g. for attraction = 6, distance must be 0, otherwise returns false
e.g. for attraction 50, distance can be between 0 and 22 tiles, otherwise returns false
