The simplest way to change video size is by FFmpeg.Conversions.FromSnippet.ChangeSize:

string output = Path.ChangeExtension(Path.GetTempFileName(), ".mkv");
string input = Resources.MkvWithAudio;

var conversion = await FFmpeg.Conversions.FromSnippet.ChangeSize(input, output, VideoSize.Nhd);

IConversionResult result = await conversion.Start();