In .Xresources:

#if NUM_SCREENS == 1 && WIDTH <= 1400
computer.type: laptop
#else
computer.type: desktop
#endif


In .config/i3/config:

set_from_resource $computer_type computer.type
include $HOME/.config/i3/$computer_type.conf


put specific settings in either ~/.config/i3/laptop.conf or .../desktop.conf