Anybody ever used a 5'x8' flat tarp with a net tent like the MLD Senenity Net Tent or Yama Mountain Gear? I am curious if the 5'x8' is just too small. Any advice?

That is too small a Tarp for a Net Tent.

1] The reason is because a 5x8 tarp would typically require a Bivy if it was raining (i.e., to protect against wind blown rain)

2] If there is no risk of rain then you don't need the Tarp and just a Net by itself would work.

3] A 5x8 Tarp would be too small IMO to maneuver under to make much use of a Net tent.