If you want to set the define tiff:tile-geometry you should use the following code:
I am trying to include some new classes in the next release that will make setting this easier. This will become something like this:
image.SetDefine(MagickFormat.Tiff, "tile-geometry", "256x256");
using (MagickImage image = new MagickImage(source)) { IDefines defines = new TiffDefines() { TileGeometry = new MagickGeometry(256, 256) }; image.SetDefines(defines); }