When I've created minimaps for smaller to medium levels before I would use a large texture mapped to a plane for the minimap and world map. The minimap/world map camera would then view this texture from a top down view.
For larger, open world style environments though I'm facing a performance vs fidelity issue. For example, a large environment requires a large texture to fit everything in, but at the same time the texture also needs to be a high enough resolution for when the player decides to zoom in onto a specific point in the map. I can make the map high resolution with enough detail up close but this creates quite a large texture size. Or I can make a lower res map which is more efficient but it becomes blurry/pixelated when zooming in.
Is this just a balance that I have to play with, or is there a better way to handle this? For example, creating a simple 3d mesh whose shape would act as the map. Or instead of using a texture using a vector graphic. How is this usually done?