I can set the resolution I want in Settings > Display. However, I want to know how you can set the default resolution of Safe Mode.
I have not tested this, but is appears to be possible. I found this information on this thread.
If Safe Mode is invoked, a new configuration key appears in:
After Safe Mode has been started, another key is created, with the resolution that Safe Mode opens with in subkeys \00 and \00\00.
If the display resolution is changed by right-clicking on the desktop and manually selecting the optimum resolution for the display in safe mode, the values in the \00 and the \00\00 subkeys change to the new applied values.
On restarting and selecting F8 safe mode again, the newly selected resolution for safe mode is persistent.
It is important to point out that incorrectly modifying the registry can result in a non working system. And modifying Safe Mode, which is often used to recover from bad settings, is even more dangerous.