James -- From your description, it sounds like you are using the new Project for the Web application rather than the Microsoft Project desktop application. Assuming that is the case, there are only three available levels of zoom in the Timeline view, which are days, 2-week periods, and months. there is no way to change this, but perhaps Microsoft will eventually add more levels of zoom for us. Hope this helps.
James -- Click anywhere in the Timeline pane that displays your Timeline view, then click the View tab to display the View ribbon. In the Zoom section of the View ribbon, set the level of zoom you want to see in the Timeline view. Hope this helps.