Well, I just keep wondering who originally said that the size of a trigger did matter (as for resource consumption).
I just tried something with an extremely huge (5000 by 5000) trigger and it did not occur to me that this was different than any other trigger.
Aside from triggers being checked regularly, that is.
I mean, each unit already has his "position" set (as a combination of x and y coordinates on a map - the way triggers are working in OFP is simply two-dimensional.
What the game does, is simply check for each unit that the trigger condition applies to, if it is within the trigger area by comparing x and y values. SO, if you have your trigger set, you always have this amount of checks to do, by checking the trigger area against a given array of units. You can further improve this by doing only one check per vehicle (not per soldier) and so on.
But, as always, dosis fiat venenum. The more triggers you have, the more checks the game has to perform.
So, if you have to use triggers, group them with specified units, if feasible
Cheers
PiLLe