I did a test to see by what percent the usable Height of my phone screen is reduced by enabling the Status and/or Title Bar:
| Landscape | Portrait | |
|---|---|---|
| Status Bar | 8% | - |
| Title Bar | 16% | 8% |
| Both | 24% | 8% |
The greatest reduction is when both are enabled in Landscape orientation.
(percentages are approximate)