The segment size would vary. From 250 up to 4000 homes passed maybe?
The only real limitation is the quality of the amplification and optics. So long as a signal can be amplified and kept within acceptable noise levels one can cover as many homes as they wish but the above is a practical limit.
Just to really cook your noodle each broadcast domain doesn't necessarily correspond to fibre, in the case of the DOCSIS 3 network its' downstream signal is split between multiple optical transmitters at the hubsite and goes to multiple fibre optic nodes.
The Cisco 10k's DOCSIS 3 output on the VM network is optical, however it feeds into an external QAM modulator which converts the digital signal into digital over RF and outputs it coaxially.
Simplified it's CMTS <Coax> Optical Transmitter <Fiber> Fibre Optic Node <Coax> Home.
The placement of the fibre reflects its' original intention when all coaxial networks were overbuilt, it is there to reduce the amount of coaxial amplifiers needed in the network, thereby reducing the amount of amplifier induced distortion on the network. Instead of a string of amplifiers from hubsite to customer a single laser replaces a load of them leaving just a few in series to get from node to customer premise.