The SPICE project aims to provide a complete open source solution for remote access to virtual machines in a seamless way so you can play videos, record audio, share usb devices and share folders without complications.
SPICE includes a channel for display auto-scaling. To use it, your guest OS simply needs the agent to be installed.
For Linux it can be instaled from repository:
sudo apt-get install spice-vdagent
For Windows you can get installer from spice-space.org.