The grid we are implementing for Saving Eden is a typical voxel grid. It is 300 units from the ground to the sky limit, and 300x300 pre-loaded (and then expanded without technical limit).
The bottom of the grid is lined with one block of solid lava rock, which is indestructible. Then, granite rock for the next 30 blocks up, and then nine blocks of biome specified rock (sandstones/dirt etc). Then, there are terrain fillers, so mountains can be higher, but the basic rule of thumb is that the lava will be about 40 blocks down.
You can build all the way up to the sky limit, but remember that lightning is attracted to the highest object! Be sure to make a lightning rod or a simple structure larger than yours, so you don't sustain weather damage.