I'm working on a grid based game/prototype with a continuous open map, and are currently considering what size to make each segment. I've seen some articles mention different sizes, but most of them is really old, so I'm unsure how well they map to the various platforms and performance demands common today.
As for the project, it's a hybrid of 2D and 3D, but for path-finding purposes, the majority of searches would be approximately 2D. From a graphics perspective, the minimum segment size would be 64x64 in the XZ plane to minimize loaded segments while ensuring full screen coverage. I figure pathfinding would be an important indicator of maximum practical size.
I realize I was a bit unclear when talking about sizes and segments.
Each segment is a cubical map of blocks, e.g. 64x64x64 blocks. These represent both geometry, but also if an actor can move into that location and is thus a potential node in the graph.
Each of these segments again combine to a larger square or cubical world map. Neighboring segments are to be loaded dynamically to ensure a seamless local map.
Therefore how many blocks fit into the screen space when rendered, sets a practical minimum for the segments, but we would probably want to have larger segments than the absolute minimum. However, since a block also represents a potential node, the segment size also affects the size of the graph, which have to be considered when choosing a segment size.