I bought a 2016 Yoga laptop with a hi-res (3200 x 1800) screen. I'm running the Windows Insider builds of Windows 10. Running a hi-res screen turns up several issues with apps that aren't prepared for all the resolution. One area I had an issue with was remote desktop which I'venow fixed.
The frst thing you do to deal with this is change Windows desktop scaling factor . Windows recommends 250% scale factor for my machine and I'm using that. The next thing to do is read a great reference from Scott Hanselman on living the hi-res lifestyle.
What I exerienced with Remote Desktop with another Win 10 machine was a small window unreadable to my 55 and over eyesight.
Doing more research, I came across this article from Falafel on Remote Desktop and Hi-Res. The tip from Falafel is to make use of Remote Descktop Connection Manager and configure the desplay settings to use Full Screen. This will scale thee remote desktop window to match your local screen and it solved my problem.
The last issue was VirtualBox. One of my remote PCs has a virtualBox VM running Slackware. After scaling the remote desktop I opened the VM and it had not scaled. After saying "hmmm", I went poking around the display settings for the VM I found the Scale Factor setting. Setting this to 200% gave me a usuable VM in a remote desktop session.