Under the hood my template uses the same PDF module, it just does some layout stuff before exporting.
The card size I'm using is 512x362 for two side by side portrait A-shaped pages (which is essentially the same shape as one in landscape). As for whether that's A4, A5 etc that ultimately depends on what size you print at. US letter is a bit different since it doesn't keep the same ratio when folded in half though. But hopefully this helps somewhat.