I can tell you that you'll have far greater results with the space blanket on the "outside” of the poncho liner. It'll block wind and moisture from passing thru your insulation, thereby making it more efficient.
As far as what size to use, I'd be tempted to use Velcro in a way that would allow you to use it as a 3\4, or fully deployed if needed. For extra insulation, simple slide an automotive windshield reflector in between the space blanket and the poncho liner.
You may also want to consider using a 2P HeetSheet from Adventure Medical. It's larger and does a better job than standard space blankets. So it can be pulled up the sides of the hammock to reflect more interior heat if needed. If it's really cold, I pull the sides of mine halfway up past my hammock liner (Clark's NF Camo) and I can really feel the interior heat being reflected.
Also, as always, make sure you stow your netting. It gathers condensation like crazy, will soak everything and freeze you out.