The big size-drivers for TX are a cockpit that can accommodate the 95th-percentile population, and a requirement to perform a specific maneuver, designed to evaluate and improve the student pilot’s performance at high g, in which 6.5 g or more is sustained through a 140-deg. turn.
The RFI states that the maneuver must start at or above 15,000 feet and end it no lower than 13,000 feet, while the aircraft loses no more than 10 per cent of its initial speed. The maneuver has to begin with at least 80% internal fuel, so that it can be performed at any time during a training sortie.
I believe, also, that the USAF wants the option to include a real radar for use when the aircraft is emulating threats, rather than a software-emulated radar that the Elbit system uses.