By volume or by total land area? There's a big difference between them.

Although it's a moot point. The writers and animators had no sense of scale, and so the distance between any two points on Lyoko is not constant, so we can't determine volume or area.

i just mean in general like is it the size of the univeres or a city?

It's impossible to answer. No combination of any two points has a constant distance, because the animators have no sense of scale. Since there's no constant lengths between points, areas and volumes can't be determined. It's mathematically impossible to determine Lyoko's area or volume.