First let me mention, that I would suggest you double check that you are 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.
Since the Forest Pack Pro/Lite 5.0.5 "All scattering limits (Distribution > Max.Density, Display > Max Items/Faces) can be disabled by setting the value to zero."
Also, it's possible to set them from Maxscript in the following way:$.maxdensity = 0
$.vmaxitems = 0
$.rmaxitems = 0
$.maxfaces = 0
You can try to use bigger patches on such a large area, just take in mind, that a "really large patch of grass" might contain a lot of polygons (which wouldn't be ideal neither). I would suggest you do some testing with the diameter about 1 meter, which should lead to about 1M - 2M objects and prepare few variations to get a nice look (most of our bundled presets are using clumps size around 30cm). There is no general rule about the size, as that mainly depends on how complex models are used, but distributing very high poly-count items might be quite a memory consuming.
Also please note, that for large scenes the recently introduced Forest LOD
feature is highly recommended (the most common use for this feature is to create Level of Detail models so that less complex models and materials can be used further from the camera to help speed up renders).
Hope that helps.