So, as we're close to the release, I decided to add this option with no UI. It 
sits in the settings file (.ganttproject in your home directory), and named 
appFontSize. Search for this tag in this file:

<option id="ui.ui.appFontSize" value="0"/>

and increase value to increase font size (or decrease it to make font smaller). 
"0" corresponds to default font size

This does nothing for me. I don't see a settings file in the home directory 
with these options 

#2 Depending on your operating system, it might be hidden or home directory 
might be not what you think it is. In particular, Windows is likely to hide 
files which start with dot in Windows Explorer interface.

I found the file and the tag. Changed the appFontSize to "10", nothing. Changed 
to "50", nothing still. Tried with negative figures to decrease the size, well, 
the only thing that changed was my confusion. I mean, I shouldn't make any 
mistake just changing the value and pressing 'save'.

Please advise. Thanks in advance!

Did you restart GanttProject after changing the value? It works fine for me.

I can't change the font size for the chart area -- only the menus and dialogs 
-- here are all my font settings from ~/.ganttproject:

 <font category="menu" spec="Dialog-plain-16"/>
 <font category="chart-main" spec="SansSerif-plain-16"/>
and
 <option id="ui.ui.appFontSize" value="16"/>

Results are shown in the attached screenshot.  I'm using Ubuntu 14.04LTS 
amd-64, and I've tried this in both 2.6.6 Brno (build 1715) and 2.7 Ostrava 
build 1 (build 1700).

Any suggestions?  thanks.

#6 This setting <font category="chart-main" spec="SansSerif-plain-16"/>
only affects labels which are drawn next to task bars. There are also settings 
for the font which is used in the timeline, and, unfortunately, they sit in 
another file hidden inside plugins 
(plugins/net.sourceforge.ganttproject/data/resources/chart.properties in case 
if you're curious)

This is a mess, of course, and we need to make it over. However, it is out of 
scope of this ticket. I create issue #956 to track this issue.

Closing all tickets with status "FeelFreeToTest". Feel free to reopen if the 
issue is still reproducible.