What scale should I use?

I want to make a game about big but lightning-fast robots. What scale is better for this task? Should I stick with human sized robots, and scale assets down, or it’s better to have real size of everything? It’s going to be around 6-8m tall robots that can move up to mach2 speed for a short amount of time and at ~300-400km/h in average. But I’m going to use some physics and want it to look realistic to the scale. There is possibility to human sized player controlled characters to appear too. And I don’t want to exclude possibility of making it all work on VR(here I mean parallax sense of scale and depth). Keeping that all in mind, what scale I better to choose to have optimal performance, feel of scale, speed and still have minimum troubles with assets preparations? Will there be a problems related to scale and range like rounding errors for huge coordinate numbers(especially in shaders) and such? Possible network problems?

Scale shouldn’t affect the engine much… number of polygons, materials shaders complexity, physics complexity and so on will affect framerate the most.

I’m not exactly framerate most concern about. I’m mostly worried about if I go too far from starting point, will there be issues with coordinate values rounding? Or some physics problems on such speed, like things phasing through each other, or something like that. I mean can there be possible problems related to scale and speed, that can appear on later stages of development. May be someone already tried something like that and knows the best practice for such cases?

If your map for the game stays below 20Km x 20Km in size you shouldn’t have any issues even with multiplayer as far as I know. For singleplayer the limit should be far higher even without using world compositon, that anyway should be used for large maps to mantain a good framerate and reduce memory usage.

Things will look better in real scale because of lighting (PBR) and stuff like physics assets/physics looking heavier in general.