Our last camping trip, we used the Advanced Elements Solar
Shower (2.5 gallons)
We would fill it most of the way full and then add a little boiling water to make it warm enough to shower with. It was in January, so the sun wasn't warm enough to heat the shower water during the day.
2.5 gallons is about enough for one person (even washing hair) as long as you turn it on and off with the handy little sprayer. It's 16 bucks on Amazon, but we got ours at Sportsman's Warehouse because we could just walk in and buy it (even though we paid more).
Don't get the larger size of solar
shower (i.e. 5 gallons). It takes FOREVER for them to heat up with the sun AND they weigh so much that it's nearly impossible to find a good spot to hang them.