Well ... instead of bitching around, I could as well just post the workaround I hear you say? Right you are.
If you can't increase or decrease the brush size in FPSC:Reloaded and you have your keyboard language set to anything different than UK English or US English, you might want to try the following:
Go to your Control Panel
Pick 'Region and Language' (whatever this might be called in your native tongue)
Go to the "Keyboard and Language" tab
Click on "Change Keyboard" (no worries, not as bad as it sounds)
Click on "Add..."
Find the "English UK and Northern Ireland" entry and check it.
Then confirm all the "OK"s
On your taskbar you usually should see the language setting of your keyboard (e.g. "DE" for German or "FR" for French)
When you start FPSC:Reloaded, click on that entry in the taskbar and change it to "EN"
Then you can change the brush size with the keys shown below. No matter what symbol you might have on your native keyboard, it will be '-' and '+' for now as you set your windows keyboard language to English.
But yeah. I also hope they will fix that so it will work for all keyboard languages.