Com base na solução <@a> do @Oposum, adicionei um "redimensionamento rápido":
# Resizing windows by 10 in i3 using keyboard only
bindsym $mod+Ctrl+Shift+Right resize shrink width 10 px or 10 ppt
bindsym $mod+Ctrl+Shift+Up resize grow height 10 px or 10 ppt
bindsym $mod+Ctrl+Shift+Down resize shrink height 10 px or 10 ppt
bindsym $mod+Ctrl+Shift+Left resize grow width 10 px or 10 ppt
Então no meu ~/.i3/config
eu tenho:
# Resizing windows in i3 using keyboard only
# https://unix.stackexchange.com/q/255344/150597
# Resizing by 1
bindsym $mod+Ctrl+Right resize shrink width 1 px or 1 ppt
bindsym $mod+Ctrl+Up resize grow height 1 px or 1 ppt
bindsym $mod+Ctrl+Down resize shrink height 1 px or 1 ppt
bindsym $mod+Ctrl+Left resize grow width 1 px or 1 ppt
# Resizing by 10
bindsym $mod+Ctrl+Shift+Right resize shrink width 10 px or 10 ppt
bindsym $mod+Ctrl+Shift+Up resize grow height 10 px or 10 ppt
bindsym $mod+Ctrl+Shift+Down resize shrink height 10 px or 10 ppt
bindsym $mod+Ctrl+Shift+Left resize grow width 10 px or 10 ppt
Como @Oposum disse: Salve e reinicie o i3 ($ mod + Shift + R).
Espero que este pequeno acréscimo possa ajudar também.