Is it possible to change the font and button size of the format bar in numbers '09
Depending on your hardware, you can change the Display preference to alter the resolution which indirectly controls how large the graphical elements on screen are drawn. This doesn't target just the numbers format bar, but affects all text and elements on the screen.

With a retina Mac, you have more control over this with the text and items being drawn with retina precision, yet the overall scaling of the graphical elements can be made larger or smaller to your liking. This is basically an implementation of resolution independence for those hardware displays.

If you need to target just the format bar, you can only choose to display or not display text and choose the default or the smaller icons using this control in the preferences for the toolbar.