import ij.IJ;
import ij.ImagePlus;
import ij.ImageWindow;
import ij.ImageCanvas;
import java.awt.Dimension;

zoomLevel = 0.2;

win = IJ.getImage().getWindow();
newWidth = (int) (zoomLevel * win.getImagePlus().getWidth());
newHeight = (int) (zoomLevel * win.getImagePlus().getHeight());
newSize = new Dimension(newWidth, newHeight);
win.getCanvas().setDrawingSize(newWidth, newHeight);
win.getCanvas().setMagnification(zoomLevel);
win.pack();
win.toFront();