So, so so so so so so so. We don't want goblins coming after fortresses with little value, we don't want goblins attacking a vastly superior force. However, there must be a mechanism to explain how they might know about your fortress. Well, how does information flow in a medieval style world?
Politics is one: the collection of tribute up the ol' feudal system ladder. Tax collectors and nobility and such would all be rather intricately connected, with couriers and similar folk the world over. Also, diplomats and negotiators would be rather well informed of goings-on. Goblins, however, would have no access to these sources of information, seeing as they are shunned from all 'civilized' races that dwarves might have contact with. They could still intercept couriers and such, torturing and bribing them for information, but well-guarded and morally upstanding people would be significant impediments.
Another possible avenue of information would be merchants: not exactly the most honorable of folk, it is conceivable that many could be corrupted to take goblin bribes, at the risk of being found out and executed for treason. Also feasible is that merchants could act as citizen-level couriers, carrying mail from the peasantry to their families abroad. Such mail could be intercepted and would be filled with information about the happenings at any fortress, but it is doubtful that the goblins would have the literacy required to understand any form of written correspondence. Far more likely is that the goblins would 'inspect' the outgoing shipments of goods for quality and quantity and base their assumptions on that.
Lastly, there remains a single other possible form of information transportation: emigration. While immigration is already quite well implemented, no fortress has yet had a single member leave out of disgust at living conditions, out of sadness, or even out of a lack of food. Still, such a system would provide a means of keeping the people's happiness at the forefront of fortress management, but that is for another suggestion thread. Let it suffice to say that emigrants would be a far more valuable source of data than any other, were they captured by goblins. And from each successful emigrant's newfound home would blossom many tales of your fortresses wealth and strength at the time of their having left.
In any case, this sort of micromanagement of data about your fortress would lead to such an extraordinary processor strain as to melt it, dragging FPS to single digits without a pair of cats in sight.