List<Size> sizes = parameters.getSupportedPreviewSizes();
Size optimalSize = getOptimalPreviewSize(sizes, w, h);
parameters.setPreviewSize(optimalSize.width, optimalSize.height);

private Size getOptimalPreviewSize(List<Size> sizes, int w, int h) {
 final double ASPECT_TOLERANCE = 0.05;
 double targetRatio = (double) w / h;
 if (sizes == null)
 return null;
 Size optimalSize = null;
 double minDiff = Double.MAX_VALUE;
 int targetHeight = h;
 // Try to find an size match aspect ratio and size
 for (Size size : sizes) {
 double ratio = (double) size.width / size.height;
 if (Math.abs(ratio - targetRatio) > ASPECT_TOLERANCE)
 continue;
 if (Math.abs(size.height - targetHeight) < minDiff) {
 optimalSize = size;
 minDiff = Math.abs(size.height - targetHeight);
 }
 }
 // Cannot find the one match the aspect ratio, ignore the requirement
 if (optimalSize == null) {
 minDiff = Double.MAX_VALUE;
 for (Size size : sizes) {
 if (Math.abs(size.height - targetHeight) < minDiff) {
  optimalSize = size;
  minDiff = Math.abs(size.height - targetHeight);
 }
 }
 }
 return optimalSize;
}