High Speed Signals PCB guidelines
If you have a vacuum-tube computer with 100 nanosecond edges, then 10nanoSec or 20nanoSec round trip delays and reflections will be hidden in the edge waveform.
In that case, your computer can be 5 feet across (5 + 5 nanosecond round trip) to 10 feet across (10 + 10 nanosecond round trip).

This vacuum-tube computer will have UGLY waveforms (lots of reflections) with 50 foot distances (100 nanosecond round trip) unless you provide some method to absorb the fast energy. Your long cables may couple into other long cables, with fast edge energy being the best transferred between wires; in that case, your signals (your logic levels) will show lots of trash coupled onto the "0" and onto the "1" levels, and you'll have slow edges because of the wire-wire transfer of fast energy; the slow edges may be clean, because the fast energy was stolen by adjacent wiring.

If you build a computer with 100 picosecond edges, you'll see two effects. (1) the FR-4 epoxy-fiberglass insulation becomes an absorber of your high-speed energy, and your fast edges become slow, thus reflections are less of a bother. (2) the fast edges will not have time to fully penetrate the copper foil, and the thinner region used by the fast energy results in higher resistance and lots of heating losses; again your fast edges become slow, and reflections are less of a bother. { using loop inductors on both sides of single-layer PCB foil, I observed about 150 nanoSeconds propagation delay; Jackson (E&amp;M book) agrees/predicts this much delay THRU the foil. }

In between the cable-bundle-losses and the FR-4/skin_effect losses, you should use Roker Pivic's rule.