Hi!
Well tom I think that in MCUs Like 89C51/52 the major factor on which length of PCB track depands is operating frequency.
For example if you want to just switch a transistor driven relay than it could be far much longer until and unless V=I*R will not produce significant voltage drop.
The reason which I think is that as track length increases the inductance of track and capacitance with respect to ground is also increases. These Inductance and capacitance is actually gives significant signal drop at higher switching frequencies.
I have once made a project in which a single 89C52 TXD pin transmit data to 3 diffrent 89C52 RXD pins with out any buffer. The wire length of each connection was nearly 4.5 feet long and the baud rate was 300 bps.