Path:
C:\Users\{username}\AppData\Roaming\Microsoft\Windows\Hyper-V\Client\1.0
The configuration of each VM is stored in a vmconnect.rdp.{uuid} file which is an XML document containing various settings including
<setting name="DesktopSize" type="System.Drawing.Size">
<value>1440, 900</value>
</setting>
Those values can be modified for non-standard resolution sizes