Maybe you have heard about the great WRLD project, which provides a great way to display real world map data in your project. Furthermore they provide several different SDKs to access these datas.
For a small project I needed some map data to visualize them in a 3D scene inside Unity. Using the Unity SDK from WRLD you can easily access those data and they will be displayed in your scene. Sadly they render the map all over your scene and their is no restriction in size. At least I haven't found any, even if you use their script attached to a GameObject with a specific size, the map will be displayed all over the scene.
After some fails and searching the web I stumbled upon a video showing the usage of WRLD in a AR environment. There they do exactly what I needed. Luckily the video was made by WRLD and they also provided two very good blog posts where they explained how they have done it. With the help of these blog posts I have implemented it without all the AR stuff and came up with the proof of concept you can see in the animation.
The displayed cube is used as a stencil mask for the map and if you move the cube or the map, only the part of the map which is inside the cube will be rendered. Also new tiles of the map are loaded dynamically depending on the main camera in the scene. I have published the Unity project on GitHub to provide the solution ready to use for your project and also to archive it for myself. You will need a valid API key from WRLD, just register at their website and generate one for your needs. Then insert your API key at the WRLD Map GameObject:
The project includes the WRLD Unity SDK which you also find in the Asset Store of Unity. But be careful if you replace the included one with the official one from the store, because I have done some changes they have mentioned in their blog posts. So make sure to apply the code changes if you replace the integrated WRLD SDK.
Hope you will find it useful. If you find a bug or have useful hints then let me know, because I'm quite new in Unity and thankful for anything related to it.