You could resize your input image an put all the images in a MagickCollection and write that to your output file. But it is easier to use a builtin feature of ImageMagick for that.
This will write an multi-sized icon that contains the dimensions 64x64, 48x48, 32x32, 16x16. Make sure your input image is square already.
using (var image = new MagickImage(convertImagePath)) { image.SetDefine(MagickFormat.Icon, "auto-resize", "64,48,32,16"); image.Write(convertedFileName); }