On Your phone
Settings > About Phone > Build number > Tap it 7 times to become developer.
Settings > Developer Options > USB Debugging.
On Your PC or Mac
Install ADB (http://developer.android.com/tools/help/adb.html)
From your shell/console digit adb shell wm size widhtPXxheightPX
Nexus 5, 5X and 6P example resolution:
adb shell wm size 1800×3200
adb shell wm size 1980×3520
adb shell wm size 2025×3600
adb shell wm size 2160×5120
adb shell wm size 2250×4000
adb shell wm size 2646×4704
adb shell wm size 2880×5120
Xiaomi mi mix example resolution:
adb shell wm size 1370×2600