I figured it out. Mach3 autosizes only to the size the screen was at when previously shutdown. If I maximize the screen, the screen elements do not automatically enlarge. However, if I then exit and restart, mach starts up with the screen maximized and the elements enlarged to fit.
IMO it would be better if the auto-size operation were to occur immediately any time the screen is resized by the user, but as I always run fullscreen it doesn't matter that much to me. The issue popped up when I reloaded to a newer version, and the initial default(?) size was less than a full screen.