Hi,
You can't resize the title template from within iMovie.
As a workaround, you can create the title in a new project for that purpose. Place the title in the main timeline and put in your text, adjusting it with spacing so that it is narrow enough to fit your 4:3 ratio. Then share it out and import it back in to the timeline. Delete the old title. Now click on the project media tab in the upper left sidebar of the My Media screen, so that the title clip displays in the media browser. Select it in the media browser. Click on the crop tool in the upper right of your screen (the one that looks like intersecting angles.) A little box with the work "Crop" in it will appear above the preview screen to the left. Click on Crop. Adjust rectangles will appear in the viewer screen that will let you adjust the dimensions of the title clip to anything you want. Adjust it to fit 4:3. Then click on the blue button in the upper right of your screen to apply the changes. Drag the cropped title clip into the timeline. Delete the other one. Share out the project. Drag the shared title into your original project.
Sounds a bit complicated, I know. If you need to overlay the title onto a video, you would need to create the title on a green background and overlay it on a green screen.
-- Rich