I've seen a requirement that they ICAO RFSS Category 4
I do confirm, it's the minimum legal RFFS requirement for ETOPS altn.
I don't see a runway length requirement.
ETOPS altns need to be suitable apt, which means to be adequate and with forecast wx within ETOPS planning minimas.
Here more infos.
As reported at all in that link would add only that planning requirements for ETP Minimum Fuel ceases during the flight, so it's legal to see fuel prediction at ETP below minimum planned.