Desktop PC detected as Phablet

Sounds like you're trying to use XMir.

Can you please attach a copy of your:
/var/log/Xorg.0.log

?

Xorg.0.log (with uninstalled fglrx drivers i think)

With installed fglrx drivers.

I've installed it using:
sudo apt-get install unity8-desktop-session-mir

I have connected monitor with DVI cable, purged unity8-desktop-session-mir, reinstalled it and logged out and tried mir, but I have same problem. In settings (in Unity 7) it now detects my monitor as 22" (actual size) instead of the 7" .

Does the problem persist if you remove unity8-desktop-session-mir?

I thought the problem sounded like XMir but your logs say XMir is not in use:
(WW) "xmir" is not to be loaded by default. Skipping.

Also a second question: Can you please run "xrandr" while experiencing the problem and paste the output?

1. Problem "In system settings (on X server) it says that my 22 inch monitor is 7 inch monitor." is still present if I remove unity8-desktop-session-mir and In ubuntu Gnome everything is scaled up. I'm currently using Ubuntu Unity 14.04 fresh install, not ubuntu Gnome (if that matters).

2. I can't run terminal in the unity mir, but this is what i get when i run it in the Unity 7:

paulo@laganini:~$ xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
DisplayPort-0 disconnected (normal left inverted right x axis y axis)
HDMI-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 160mm x 90mm
   1920x1080 60.0*+ 50.0 59.9 30.0 25.0 24.0 30.0 24.0
   1920x1080i 60.1 50.0 60.0
   1600x1200 60.0
   1680x1050 59.9
   1280x1024 75.0 60.0
   1440x900 59.9
   1280x960 60.0
   1280x800 59.9
   1152x864 75.0
   1280x720 60.0 50.0 59.9
   1024x768 75.1 70.1 60.0
   832x624 74.6
   800x600 72.2 75.0 60.3 56.2
   720x576 50.0
   720x480 60.0 59.9
   640x480 75.0 72.8 66.7 60.0 59.9
   720x400 70.1
DVI-0 disconnected (normal left inverted right x axis y axis)
DVI-1 disconnected (normal left inverted right x axis y axis)

OK. That's a third point of proof that you're not running XMir. You have an interlaced mode "1920x1080i" which is not possible with Mir. So this can't be a Mir or XMir bug.

The X log files say it's "radeon" misdetecting the display size. So I'll reassign the bug to that...

[ 21.327] (II) RADEON(0): clock: 27.0 MHz Image Size: 160 x 90 mm

As I said I ran that command in Unity 7 (on XServer because I can't open terminal in the Unity Mir.

Well "160 x 90 mm" is obviously too small so there's a bug somewhere. Do those dimensions change if you remove all the *mir packages?

I've already reported that problem:
https://bugs.launchpad.net/elementaryos/+bug/1356996

So, I've installed ubuntu-terminal-app and ran xrandr from the terminal and this is whatI've got:
http://oi62.tinypic.com/2rvzbwp.jpg

Sorry for the delay. This bug got lost in the sands of time.

Is there still any issue?

yes. That probblem affects both proprietary and open source AMD drivers on wayland, x and mir when I connect monitor to GPU with HDMI cable. It appears on KDE, GNOME, Unity and KDE.

Sorry, we should have noticed this earlier...

It appears you have a buggy monitor which reports itself as 16x9cm in size when plugged in to HDMI. So that's why Unity8 thinks it's a tablet. Although you also mention in comment #5 that if you connect using DVI then the dimensions are reported correctly. So you should use DVI as a workaround if you can.

So we have a hardware bug in the physical monitor, and a software bug in Unity8. Unity8 is incorrectly assuming that a 16x9cm display must be a tablet. Actually we can correct this assumption by noticing it's connected by HDMI and therefore is not a tablet display.

We probably can't fix the issue with Gnome scaling everything up. In that case the software is doing the right thing, based on the incorrect dimensions that the buggy monitor is providing. So again, try and see if using the DVI connector avoids that EDID signalling problem.

Also note: Buggy hardware that reports incorrect EDID information is very common. You're not alone. So Unity8 will need to account for this in future and allow the user to override the current form factor.

FWIW, unity8 doesn't care about the EDID at all and determines the use of staged vs windowed mode depending on some other factors, like attached input devices and size of the actual painted surface.