Zoom

Controls how tall each hour is on the Day/Two Days/Three Days/Week time grid. Only shown in Calendar, and only affects those time-grid views -- Month has no concept of zoom, and Kanban doesn't have this control at all.

Button	Effect
Zoom Out	Shrinks the grid one step.
Actual Size	Resets the grid to its default zoom level.
Zoom In	Enlarges the grid one step.

You can also zoom with Ctrl/⌘ + scroll anywhere on the time grid, and reset to the default with a middle-click on empty grid space (not on an item).

There are 5 zoom steps between the smallest and largest size, with the default sitting in the second step. Like the other display options in the toolbar, zoom level isn't saved -- it resets to the default the next time you reload the page.