Hi, although Forest can handle large areas, there is a limitation in the number of items to scatter.
We suggest to keep this limit below a few millions of items. Above this value, memory and performance hit may be significative, not only in Forest but also in the render engine.
Said this, the procedure to scatter large areas is very simple:
1) Select one of the grass presets from the Library (large mode).
2) Apply it to the surface / spline.
3) Turn on "Camera->Limit to visibility" and assign your main camera. This wil save some resources from the non-visible items.
4) Render. If you get this error
, increase the Density limits as described in the link.
What is your terrain size ? What error do you get exactly ?