I don't know if Ubuntu detects the available resolutions but I know that VirtualBox can resize depending on machine resolution. The ideal resolution would be 1900x980 but I tried others too (like 1792x960 and 1728x972) and nothing works. Please help, thanks!
I solved it now that I saw "View > Auto-resize Guest Display" on VirtualBox machine window (the "xrandr" confirmed it) but maybe it requires "VBoxGuestAdditions.iso" installation if just click view-auto don't work.
rher@u22:~$ xrandr
Screen 0: minimum 1 x 1, current 1920 x 977, maximum 8192 x 8192
Virtual1 connected primary 1920x977+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
1920x977 60.00*+
2560x1600 59.99
2560x1440 59.95
1920x1440 60.00
1856x1392 60.00
1792x1344 60.00
1920x1200 59.88
1920x1080 59.96
1600x1200 60.00
1680x1050 59.95
1400x1050 59.98
1280x1024 60.02
1440x900 59.89
1280x960 60.00
1360x768 60.02
1280x800 59.81
1152x864 75.00
1280x768 59.87
1280x720 59.86
1024x768 60.00
800x600 60.32
640x480 59.94
Virtual2 disconnected (normal left inverted right x axis y axis)
Virtual3 disconnected (normal left inverted right x axis y axis)
Virtual4 disconnected (normal left inverted right x axis y axis)
Virtual5 disconnected (normal left inverted right x axis y axis)
Virtual6 disconnected (normal left inverted right x axis y axis)
Virtual7 disconnected (normal left inverted right x axis y axis)
Virtual8 disconnected (normal left inverted right x axis y axis)