Ethernet can run up to 500m on a half-duplex link (thick coax). There is a limit for the total network diameter to ensure that collisions are still detected, but there's no way you're going to exceed it with a single twisted pair link.
I should have clarified that the 100 meter limit is for 100 megabit and gigabit Ethernet. For 10 megabit Ethernet the limit is 500 meters
per backbone segment, but you can go something like 2500m (IIRC) end-to-end, through repeaters. It was designed so that you could have a 1500m fiber segment bridging between two "ethers."
Hubs are always half-duplex. Switches can be either half or full duplex according to either the port settings or autonegotiation. One of the most annoying problems I've encountered is admins forcing the ports to full-duplex which breaks autonegotation. The end result of this is terrible performance unless you set the network card in the PC to also run in full-duplex.
Unmanaged switches will not allow you to change the port duplex settings manually. They are autonegotiation-only.
Yes, you are quite correct that switches can support half-duplex. I was thinking in terms of going switch to switch for the OP's situations, but I did omit that most switches can operate either way.
