My RPi2 normally runs with my Philips V-Line 196V4LAB2 18.5" DVI-D monitor which is 1366 x 768 Pixels and runs in hdmi_group=2, hdmi_mode=86. Raspbian handles that with no troubles and it automagically picks an ideal mode.
To get anything other than scrambled fuzziness with Win10 IoT I've currently coded hdmi_group=2, hdmi_mode=4 (I found a config.txt on /dev/sda1 when I mounted my IoT SDCard on another RPi) but now the main screen is truncated so only the top left of the screen shows.
What's the minimum size needed to get the full picture on the screen? Because I'm going to need to reboot Raspbian to see what other modes are possible with my DVI-D monitor. (I've had the same problem with FreeBSD when I tried that.)