I camped at Onion Valley campsite 19 last month, and while more spacious than some of the other walk-in sites there is not a lot of level ground. There is no designated tent pad but there is a pretty obvious flat spot to pitch a tent. I had a 2 person tent and it was just about the optimal size. While there is room for a bigger tent, the floor might not be perfectly level. It looked like the people camping at space 21 had a 3 or 4 person tent and while it fit it took up whatever space was left on the campsite after the bear locker and picnic table. I really liked Onion Valley and the warmup hike I did on the Kearsarge Pass trail. Hope this helps.
