My timeline is suddenly micro sized - almost to the point where I cannot see the controls or the video. I've seen the post recommending double clicking on the vertical bar on the right and this makes it slightly larger but still far smaller that it was. What can I do to reset this?
Place your cursor on the bottom of the track header. The cursor will change shape. Hold down the left mouse key and drag the bottom of the track header down. I'm not at my editing computer but I recall that you can also increase the size of the track by double clicking on the track header.
In addition to what JackW writes, note that the Minimize/Maximize/Restore Track height buttons in the Track header are in the "Hamburger" as of v.15 and you need to "Edit visible button set" to see them. See also what the Track height controls (+ and -) under the vertical scroll bar do. Also, "Press Ctrl+Shift+Up/Down Arrow to increase or decrease the height of all tracks."