The strongest driver on the size of the LUVOIR telescope is the desire to have a large sample of exoEarth candidates to study. This figure shows the real stars in the sky for which a planet in the habitable zone can be observed. The Sun is at the center.
Most of the accessible stars are solar-type FGK stars. The color coding shows the probability of observing an exoEarth candidate if it’s present around that star (green is a high probability, red is a low one).
This is a visualization of the work of Chris Stark at STScI, who created an advanced code to calculate yields of exoplanet observations with particular facilities.