The largest font allowed for the center bars and bottom links is 10 points. The reason is that the height of the area is only 17 pixels, 15 pixels for Bottom Links.
This cannot be changed simply by creating larger images in the Override subdirectory. This requires a program/layout change and is not available at this time.
The are 2 checkboxes on the Advanced List Options for Larger Font and Larger Font (Bottom Links). Selecting these checkboxes indicates you want the 10 point font. Unchecking these boxes specifies to use the 8 point font.