option 1: via SVP's frame size controls - there's no UI button "decrease to 1080p" but you can set frc.frame.resize in 'All settings' to "-19201080". "decrease to screen" should also work if your screen resolution is 1080p.
option 2: use any of ffmpeg's scale filters in 'lavfi' field in transcoder's 'Advanced' mode - the trivial example is "scale=w=1920:h=1080"