That's an interesting question, and although there's not a built in way to do that with Forest Pack, there might be a workaround that'll be suitable for your needs that uses a Forest Effect.

Please take a look at the attached file. In it, I've actually scattered the items on the stairs rather than the ground. A forest Effect then finds the distance between an items position on the stairs and the FP pivot (which is placed at ground level). It then divides that by the height of the geometry to find a scale multiplier and then uses that to adjust the object's size on the Z Axis. In order for that to work you'd have to create the Forest Object in a particular way:
- Scatter the objects on the stairs rather than the ground
- Move the Forest Object so that it's pivot is on your ground plane.
- In the Distribution rollout, set the Collisions Height to 100%. You may need to
temporarily enable collisions to edit that parameter. This makes the size available to effects, but you don't actually need collisions to be enabled for effects to get the value.
- In the effects rollout, load the attached Forest Effect. It has two parameters for adding min and max scale variation. If you don't want variation, set them to the same value.
That's it. I hope that helps.
Paul