iMovie font and size default

Make sure the title appears in the Viewer and double-click it in the Viewer so that it is selected as in the screenshot below.

When you have done this the controls for selecting fonts and sizes will magically appear as shown by the 2 red arrows.

Alternatively you can bring up the controls by hitting the large "T" at the top left above the Viewer.

Just in case you don't know, you have to drag the chosen title down to the timeline before you can get access to the font controls.

The title can go straight into the timeline itself or can be dropped above a clip already in the timeline.