Looking through your amazing photos of this zoo, it seems very nice! I have the same question as snowleopard, though are the howlers kept in a smaller portion of the exhibit for easy spotting?
I did an attempt at measuring the approximate size of the squirrel monkey/red howler monkey forest in Google Earth, but because of the dense forest cover of the area I couldn't see much of the borders. My best guess is that the area is roughly 4.000 square meters.
The exhibit is basically an area of pre-existing Dutch forest that they adapted, with a moat and fencing around it, pathways and a few shelters and buildings, to serve as a large walk-through exhibit with two species of monkey and the Darwin's rheas, who seem to mostly hang around in the clearing/open area near the shelter where their stable is.
It is quite a lovely exhibit, I agree, and it is also quite popular with visitors. And it is nice to see monkeys in actual, albeit Dutch, trees and bushes rather than just manmade versions of those.
As far as the visibility of the monkeys goes... given the dense foliage (at least in late spring, summer and early fall), the size of the area and the height of the trees, you can probably understand that they can in fact hide completely out of sight and that locating and spotting the monkeys (both species) can be a bit of a challenge.
I believe, however, if my memory isn't letting me down, that I have seen both the squirrel monkeys and the red howler monkeys during each of my visits, at least distantly or high up in the canopy. Seeing the animals up close and making decent photographs of them can be another matter (for example during my most recent visit I didn't get any successful photographs of monkeys in the forest). So I'd say locating the monkeys in the forest can be a bit challenging, but it is also far from impossible. I hope I've answerred your questions.
