It's more a question of how much space you want to devote to them.
The size of the decoy depends more on how much you want to increase the chance that your opponent's missile will hit the decoy and not the ship.
This is determined by the signature of the decoy the bigger it is the greater the chance it will shield the ship.
In your case, if one decoy is launched.
The chance of the missile hitting the ship is:
15000/(15000+1600)=90,36%
Without taking ECCM and ECM into account.
Minimal Decoy Size is 5.
The only issue is how many decoys will be launched.
Where you have 1600 in the formula, this is the total signature of the decoys fired.
Read this post:
https://aurora2.pentarch.org/index.php?topic=13407.msg167258#msg167258And here is an oryginal change log post:
https://aurora2.pentarch.org/index.php?topic=13090.msg164759#msg164759