For 100baseT as a shared media (ie: using a hub), there is a hard limit due to timing. There is also a limitation on the number of hubs in the network, again due to timing issues.
The basic problem is that the frame must be able to reach the furthest point in the network and return before the originating station has finished transmitting the frame. The timing is based on the minimum frame size of 64 bytes.
The rules effectively limit a shared 100baseT network to a single hub driving 100m segments, or two "class II" hubs driving 100m segments, separated by up to 5m of cable linking the hubs.
I should mention that ethernet has been tested successfully over short lengths of barbed wire.
I should amend my posting above. IEEE 802.3 notes that the non-populated link segments are supposed to be full-duplex (twisted pair or fiber), and the length of the AUI cables used with fiber transceivers is 25m.
