Is saving weight important? If not then grab any old silnylon tarp. If solo you'll want something around 9x7 and if planning to camp with someone then you'll want something like 10x8>>1386130>out a lot of campsites depending on weather
That's sort of the deal with cowboy camping. It's a bonus for when conditions allow it. I'm not saying don't cowboy camp unless you are super high, but if it's going to be a dry night then pick the highest site you can. Beyond that >>1386149>condensation is pretty much unavoidable
You can carry a bivy bag if you are committed to camping without a shelter. It doesn't have to be a heavy waterproof one, borah gear make a really nice light bivy.
You can also do a few more things to help deal with dew. Avoid camping on veg, under trees is always good as there is something of a micro climate that is drier than say a grass field.
If you can't camp under trees and have to sleep on veg then think about carrying a decent sized piece of window insulation film, it's real light, packs down super small, and sleeping on that is going to keep the moisture from the veg underneath.
The thing to rememeber is that sleeping out without a shelter either requires some sort of a bivy or the prefect conditions. You could also switch over to a synthetic bag as it'll handle the wet a lot better. I use a synthetic quilt during the 2.5 seasons as the higher temps allow me to carry a smaller and lighter one. Plus I very rarely cowboy camp into the shoulder season.
Pic is my setup for sleeping without a shelter.