Recalbox is likely detecting your display in portrait mode by default. To fix this, edit the recalbox.conf file by adding or modifying the line:
global.videomode=custom
Then, edit config.txt and add:
hdmi_cvt=2560 1440 60
hdmi_group=2
hdmi_mode=87
display_rotate=3
This forces the correct resolution and rotates the display. Restart Recalbox to apply the changes.