Using the surface like it's in the provided scene shouldn't lead to any scaling issues, but checking your scene, I've noticed that items used in the Forest012 are using the Global Scale 50% (attaching a screen-shot).
Please double check, you have the Respect System Units in Files option enabled in the System Unit Setup window and the Rescale the XRef Objects to the System Unit Scale option when importing presets.
Let me add, that on larger areas, you should be using presets that are meant to cover a large area(s). These will create fewer instances and are less taxing for the renderer. For both the Lawns and Meadows presets you will see there are two versions for each type, one with a (detail) suffix and another with a (large) suffix. Unless you're filling a small area, I'd pick the large ones.