There are practical limits to the size of an Ethernet network and one of the primary concerns is the length of the shared cable.

As electric signals propagate along a cable quickly, they become weak and signals can also be scrambled through electrical interference from neighboring devices. To minimize delay and receive each other’s signal clearly, the network cable must be short enough. Thus, a distance limitation on the maximum separation on the network diameter happens.
There are also practical limits to the number of devices that can coexist in a single network since CSMA/CD is only a single device. Contention for the medium increases when there are too many devices attached to one shared segment and there will be a long delay before getting a chance to transmit.