The default screen resolution in Virtual Box for OpenSolaris is 1024×768.
If you want a higher resolution screen for you OpenSolaris virtual machine then before starting the VirtualBox guest run:
C:\virtual\box\install\path> VBoxManage setextradata "OSolarisVB" "CustomVideoMode1" "1280x10240x24"
Where “OSolarisVB” is the name of OpenSolaris VirtualBox and I’d recommend bumping the video memory to at least 64MB.
Then run in full screen mode.