Change the physical size of my screen on Gnome3

I have a monitor connected via VGA that gnome detects in 40" (885mm x 498mm) but when I connect in HDMI he detects him in 7" (160mm x 90mm).

Here are the details:

Screen 0: minimum 8 x 8, current 2640 x 768, maximum 16384 x 16384
VGA-0 connected primary 1360x768+0+0 (normal left inverted right x axis y axis) 885mm x 498mm
   1360x768      60.02*+
   1024x768      75.03    70.07    60.00  
   800x600       75.00    72.19    60.32  
   640x480       75.00    72.81    59.94  
HDMI-0 connected 1280x720+1360+0 (normal left inverted right x axis y axis) 160mm x 90mm
   1280x720      60.00*+  59.94    50.00  
   1920x1080     60.05    60.00    50.04  
   720x576       50.00  
   720x480       59.94  
   640x480       59.94  
DVI-D-0 disconnected (normal left inverted right x axis y axis)

How I could change (manually or automatically) the size obtained by gnome? If this is not possible, it would come from the cable?

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