After testing it in overnight in the garden my wife and I used one of these over night recently on a wild camp. We had it strung between two trees and found it was long enough for me at 6'4" and backpacks at our feet. It rained, though with no or very little wind and we stayed dry and so did our bags.
If it were windy and rainy we may have had to adjust things a bit. I did try briefly closing one end, this makes the sleeping space shorter, but would cut out wind and rain from one end, but if you're in a bivvy bag you might not care if your feet get rained on.
For one person I'd say you'd have plenty of space for you and your gear. A larger tarp would have made things easier getting in and out though.