public static int maxTexture3DSize { get; }

The largest 3D texture width, height and depth 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 3D texture with a size of 2048 in an uncompressed format such as TextureFormat.RGBA32, you need 32 gigabytes of memory because the amount of data is 4 bytes per pixel × 2048 x 2048 × 2048.

Additional Resources: SystemInfo.maxTextureSize, Texture3D.