First let me mention, that there is no "physical cut" performed while using the Edge Boundary Checking
mode (as this would be extremely resource demanding). The "trimmed part" is treated by the V-Ray render engine as a kind of "Matte object" internally (this way render engine ignores it for its calculations).
But, 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 to do some testing with diameter about 1 meter, which should lead to about 1M - 2M objects and prepare few variations to get 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 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.