Also, the messages your report are normal. Java does not report the known lafs properly, so I have a list in BT747 that is 'tried' in order of priority until an existing LAF is found. After that it reports the LAFs that java reports (but this list is not always complete).
You can set another LAF when adding this on the command line for the java call:
-Dbt747.laf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel
Can you try this NimbusLookAndFeel. If it works, I'll add it to my list of LAFs in BT747 so that it gets found before the GTK one does.
I added a Font Size option to the latest development version in the Output Settings screen on the left of the language setting.
Currently, the changes are not entirely visible dynamically - only the 'Panel' titles changes dynamically. You'll need to restart to see the full effect.
As that is not critical, I will probably not spend much more effort on it in the near future.
I agree this is small. I'll have a look how this can be configured - it is likely some configuration on your machine but could be nice to have configureable somehow too.