Read this helpful response to a similar question:
A 10' high environment will require extra time to reach desired temperatures and will be unlikely to heat proportionately. Additionally, it will be rather cool where you are sitting. We all know heat rises and, consequently, as the steam enters the room it will rise to the ceiling and push the cooler air down leaving you "out in the cold".
Let me strongly recommend that you reevaluate the potential for lowering the ceiling height in your steam room. Short of that, I would suggest two-tier benching to allow for a higher sitting point in the room. This will provide the bather a way to sit higher up in the room and the heat.
Have your contractor read the Ceiling Height and Slope
text in the Steam Shower Design Tool and then call a Steam Bath Specialist to finalize your project.
If you do decide to keep the 10' height, properly sizing your generator for this application is critical!