I cannot reproduce your problem with the Resize method. Are you using the latest version of Magick.NET? Does the following work for you:
The grayscale method is not yet available in Magick.NET. It will be added the next release.
using (MagickImage image = new MagickImage("logo:")) { image.Resize(2100, 1400); image.ToBitmap().Save(@"C:\test.jpg"); }