I just saw some animations of TRAPPIST-1 on a network news prg & it showed all seven planets alongside the Earth.

All of them appear to be smaller than the Earth.

Made me wonder if any of those planets could maintain a breathable atmo.
How much smaller could a planet otherwise similar to Earth be & still have an atmo thick enough for us to breathe w/o assist?
(I imagine a Venus-size planet could. But could a Mars-sized one?)