Frankly on VirtualBox, I solved my I3 issue configuring resolution by a standard ubuntu/linux environment way , because the chosen answer in here didn't work for me when using VirtualBox ('xrandr --dpi 220') - my i3 session windows was just closed.
So I used the following standard sequence of commands for configuring resolution:
cvt 3840 2160 60
// Output: -> Modeline "3840x2160_60.00" 712.75 3840 4160 4576 5312 2160 2163 2168 2237 -hsync +vsync
Using part of output (after word Modeline) as a parameter of a new command
xrandr --newmode "3840x2160_60.00" 3840x2160_60.00" 712.75 3840 4160 4576 5312 2160 2163 2168 2237 -hsync +vsync
xrandr --addmode VGA-1 3840x2160_60.00
// this last command activates the created resolution
xrandr -s 3840x2160
In order to preserve the configuration between user logging sessions, one has to put of course the following commands into a .bashrc init file.
Once configured, to get rid of 1 pixel scrollbars spaces, View->Scaled mode from menu is option. I also hat to configure my UltraHd display as a main display in Windows host.
My solution is a combination of two answers from the following link:
Edit: Simplified appliance containg one one-liner
eval $(cvt 2220 1250 60 |sed 's/Modeline/xrandr --newmode /g'|sed -n '1!p')
as a proper result resolution screen size aspect ratio might be afterwards reevaluated/adjusted, therefore find out the created resolution by xrand command - appended in the end of output,
1) assign the resolution to a specific display -
xrandr --addmode VGA-1 "2224x1250_60.00"
2) output the desired resolution on the display
xrandr --output VGA-1 --mode "2224x1250_60.00"