|What's the maximum map size that can be achieved? Is it possible to create a terrain size of lets say 50 km^2 or even bigger?|
For a short answer, please consider this screenshot. It shows a 2D top/down view of the TechDemo map from the movie, loaded into the map editor CaWE:
The highlighted square indicates the dimensions of the demonstrated outdoor terrain (40320^2 map units, about 1 km^2), while the grid indicates the maximum map dimensions that CaWE can currently handle in a single map file (131072^2 map units, about 11 km^2).
The long answer is that there are several ways to achieve much larger terrains than the above image suggests:
A very simple but highly effective approach is to design multiple
maps in a checkerboard pattern such that the outer terrain borders of each map seamlessly translate into the terrain of each neighbor map. For example, if you use a set of 3x3 maps, you can easily cover an area of 99 km^2. Automatically loading the next map when the payer comes near the border of the current map is easily achieved with a set of trigger brushes and a few lines of script code.
Also, there is no principal limitation to map and terrain size that can be kept in a single file either in CaWE or the Cafu engine itself, but using such large terrains requires the consideration of additional questions like terrain resolution, memory consumption, the details of creating and editing such terrains, etc.
It also requires some custom programming, if only for extending the maximum map dimensions that are available to the CaWE terrain editing tools and to customize all the detail issues that very large terrain bring with them.