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I know that this post is extremely old but wanted to share my solution, as I had the same issue and it might be of help to someone else, even if for a Linux guest vm(!) and not your windows 7 guest.
Upon booting the vm (arch Linux) from within windows 10 by way of a shortcut and the -X switch to fullscreen, terminal resolution remains at 800x600.
My solution was to start x using a .xinitrc entry and setting resolution there. As my specific resolution (1920x1080) is not know to X upon starting, I add it with xrandr including correct modeline and set the resolution before running my windowsmanager (xfce4):
xrandr --newmode 1920x1080_60.00 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync
xrandr --addmode Virtual1 1920x1080_60.00
xrandr --output Virtual1 --mode 1920x1080_60.00
exec startxfce4
Be sure to set correct monitor name (e.g. Virtual1) and modeline.
This solution seems to break Autorefit when resizing / toggling fullscreen mode) but since I can't get unity to work anyways, I couldn't care less.
Hope it's of use to someone!