This option wasn't available in the batch output, however we have now added it.
In tonight's 2.9 release (b598) we have added support for a new batch output column "Screenshot pixel density". Enter "1" for 1x, "2" for 2x, "4" for 4x and "8" for 8x. Leave blank to use the default setting (4x). This option can be used for the PDF, PowerPoint and Screenshot output types.