This will partly depend on how you want to camp. If you are thinking about using either a waterproof or water-resistant bivy bag as part of your shelter then a smaller tarp will suffice. If not then you might need to consider the best way to add some midge netting for the times when the bugs are about. You can generally manage with a slightly smaller tarp in woodland and similarly sheltered areas too, as you don't get quite the same problem with rain splashing or driving under the tarp edges as you do on an open hill.
I would suggest looking at the
Alpkit Rig 7 as a good base for comparison. It's usefully larger than the old GoLite Cave I've used for years, so I've a fair idea of how it will function despite not having used mine yet, and seems to be quite solidly made. For two people the size will be adequate in a few basic pitches (A-frame style, for example) but it might be a little too small to give enough room if you're pitching it in a more enclosed fashion and folding it around to form a door. The Rig 7 has plenty of good tie-out points, including some to let you put the ridgeline above the tarp to handily avoid water dripping down it and onto you.
Backpackinglight has a few good options to consider, including a couple from DD Hammocks: you can also go to the
DD Hammocks site directly; have a look at the 3x3 Tarp which should be suitable for your needs, amply big enough and very reasonably priced, though heavier than the Alpkit Rig 7 or Backpackinglight's own brand offerings.
If you're not sure about sizing the standard procedure is to take a couple of sheets and clothes pegs and construct a mockup at home to the size of the tarp you're interested in
EDIT: And Shewie, typing rather faster, points you to exactly the same options!