The world is a cube of side length 61840. Coordinates go from −30912 to 30927 in any direction.
The side length is a multiple of 80, an important number in Minetest.
773 × 80 = 61840
80 is the side length of a Mapchunk. A Mapchunk is a cube of 80×80×80 blocks which is a piece of the world which is generated by the map generator at once. (80 in turn is a multiple of 16, and 16 is the side length of a MapBlock, a cube which can be
loaded at once. This means 5×5×5 MapBlocks neatly fit into a Mapchunk. But back to topic.)
This explains why the world side length is a multiple of 80 but it doesn't explain where the 773 comes from, however. This seems like a fairly odd number.
Maybe you need to ask in the Minetest chatroom to find out more.
And yes, you can change the size of the world. But it is only possible to make it smaller.
Edit the advanced configuration setting “map_generation_limit”. Explanation:
# Where the map generator stops.
# Please note:
# - Limited to 31000 (setting above has no effect)
# - The map generator works in groups of 80x80x80 nodes (5x5x5 MapBlocks).
# - Those groups have an offset of -32, -32 nodes from the origin.
# - Only groups which are within the map_generation_limit are generated
# type: int min: 0 max: 31000
I do not recommend using this setting if you're not an experienced user. It it fairly odd to use.