public static int maxCubemapSize { get; }

The largest cubemap texture width and height that your graphics hardware supports.

Unity only supports textures up to a size of 16384, even if returns a larger size.

You can only create a texture with the maximum size if you have enough memory. For example, to create a cubemap with a size of 16384 in an uncompressed format such as TextureFormat.RGBA32, you need 6 gigabytes of memory because the amount of data is 4 bytes per pixel × 6 textures × 16384 x 16384.