There seems to be a map size limitation of 1024 x 512.
We've known of a limitation for a while, but I don't think a reason why was ever posted so here is one.
In the ctMoMove Command Packet, the waypoints have the x and y coordinates bit packed into a 4 byte field. The lower 15 bits are for x and the next 14 bits are for y. They have pixel granularity, so that gives a maximum of 2^15 / 32 pixels per tile = 1024 tiles wide, and 2^14 / 32 pixels per tile = 512 tiles high.
Outside of that range units will not be able to set proper destinations due to overflow.