I have berryboot installed, and it has it's own config.txt that overrides the loaded OS' config.txt (so modifying config.txt from the cmdline within the OS does not work). To be able to modify the config.txt file, one has to access it from the berry boot menu.
Now that I have shot down the obvious 'solution' of using cmdline (which does not work), I will present the problem: Using berryboot's config.txt file I modified the screen height and width (as I wanted the text to be reasonably readable on my 3.5 inch screen), however I found 680x400 was still not sufficiently large enough.
I decided to change the values to '340x200'. And thus my problems began. I figured the RBP would be smart enough to try to fit the window size within the borders (like it did with 680x400) but alas no. I only see 1 quarter of the screen (bizarrely, the OS menu etc are all scaled to fit but the windows for shutdown, berryboot menu etc are still oversized and inaccessible).
Effectively, all the buttons for accessing the berryboot menu are off-screen, and thus inaccessible (I can click to go into the edit window by resizing the start-up screen quickly enough but the edit window is blown up such that ALL of the window is off-screen, including the resize portions, and thus inaccessible). Remember that cmdline will not modify the berryboot's config.txt, so that's solution is out the window.
I suspect there is a way to solve this with keyboard shortcut commands, but as I cannot see the menu, I cannot see what is happening (I am also unfamiliar with any keyboard commands). If someone could give me a set of 'blind' keyboard commands to get from the berry boot edit menu into config.txt where I can easily edit back the config.txt and exiting so it saves, I'd be grateful.
I've found the claim that due to berryboot being Qt, it has limitations using keyboard shortcuts and has only four (Add OS = Ctrl + A, Edit OS = enter, Delete = del, Exit = esc), none of which allow access to the more influential config.txt. It also does not allow for tab or arrow-key navigation. Unless there is some way to locate and modify berryboot's config.txt by using some sort of command line, the only alternative is to format the SD card and re-download berryboot. Something that seems a bit excessive for simply modifying screen size.
I've submitted a bug report, because I can't imagine I'd be the only person to run into this problem (or others that might put 'config.txt' out of reach). I've tacked on a few suggestions with said bug report for solutions.