When using RDP on a high DPI monitor, setting Resolution to "fit to panel" results in using the "actual" resolution of the monitor, not respecting Windows' DPI zoom setting (e.g. at 175%). If you set a static resolution (e.g. 1920x1200) then it's not fit to window / stretched to the edges.
Windows 10 x64, Dell 2715 4k monitors (3840x2160)