How to change physical display size

Hi, I have 34’’ display with resolution 3440x1440 connected by HDMI. For some apps I need to change resolution to 1920x1080. I’ve tried in KDE settings and with xrandr but every time get the smaller visible part than expected:

xdpyinfo | grep -B 2 resolution
screen #0:
dimensions:    3440x1440 pixels (726x304 millimeters)
resolution:    120x120 dots per inch

xdpyinfo | grep -B 2 resolution
screen #0:
dimensions:    1920x1080 pixels (405x228 millimeters)
resolution:    120x120 dots per inch


I’ve tried to change DPI to 82 with xrandr, but no changes.
When I switch resolution in windows the same way - all is ok.
Any suggestions, please?

my system:
Operating System: Manjaro Linux
KDE Plasma Version: 5.20.4
KDE Frameworks Version: 5.76.0
Qt Version: 5.15.2
Kernel Version: 5.9.11-3-MANJARO
OS Type: 64-bit
Processors: 4 × Intel® Core™ i7-7500U CPU @ 2.70GHz
Memory: 7.7 ГиБ of RAM
Graphics Processor: Mesa Intel® HD Graphics 620

Hi and welcome to the forum :+1:

IMHO, you need to use software scaling if your monitor only supports a native resolution of “3440x1440”.

No idea if your graphics driver supports the same option, as what i suggested below, but you can try and see if it helps:

There is another value you could try to add to that first line using a comma in the value: NoEdidHDMI2Check
Eg. as "AllowNonEdidModes, NoEdidHDMI2Check"

Thanx for answer, but I mean situation when I have to switch resolution on the fly, without restart. because of it I’m using script with xrandr:
xrandr --output DP-1 --mode 1920x1080 --rate 60.00
I’ve tried different options with xrandr but without success, --dpi, --fbmm, --scale. Only one option gave me a visible result --fb, but I can’t find info what units used by this option, when I try
xrandr --fb 405x228
screen is very small but keeping fonts and icons DPI.

First try and tell if not work :wink: