Let's assume the Russians get what they want and completely landlock the Ukraine.
We now have 4 fronts, in which I have paid no attention to terrain features and infrastructure.
Ukrfront (red) - 1563km
Belfront (lime) - 1172 km
Baltfront (blue) - 559 km
Scandfront (grey) - 1242 km
Total size of front: 4536km