Hi,
Do you mean 81000 x 81000 pixels? You can create a terrain with this size in Instant Terra Pro, but it will require a huge amount of memory (more than 20 GB for each node).
Erosion nodes do modify the terrain, but probably not at continental scale, and works of tectonics are not simulated.
One possibility is to split your terrain into small parts, and create one project per part. Then you have a final project that blends all parts together.