Contrary to what I said above, this is not just a simple scaling issue.
I will need physical access to a real device to implement proper support for screen size other than 4.5" (for creating test cases and checking resulting user experience). So it may take some time.

BTW, do you have any suggestion for a cheap device with 5+" display running SailfishOS (or at least easy to install) ?

@claustn log files are located in ~/.local/share/okboard (*.log & *.log.bak files). I have just added this information in the settings app.