Please note: This works in Gnome 3.16 and does not work in later versions.
You will find there is no option in the settings of Gnome3 that allows you to change the font size of the top menu panel in Gnome3, however by changing the gnome-shell.css file this can be achieved.
The gnome-shell.css file can be found in the following directory /usr/share/gnome-shell/theme/gnome-shell.css
Open you terminal and enter the following command to edit it
For the changes to take effect, you will need to logout of Gnome3 and then log back in again.
I'm hoping in the future the developers will provide an option in the settings to change the font size.
I can't fault anything else in Gnome3 it is a wonderful window manager.