Use the zoom controls at the bottom right of an Idea Workspace to zoom out, zoom in, or select Fit to show every card at once. Select the percentage to go back to 100%.
You can also pinch on a trackpad or hold Ctrl while you scroll, which zooms around the pointer. The keyboard shortcuts are the same as in Design: + and −, Shift 1 to fit, and Shift 0 for 100%.
A workspace now opens zoomed so all of its cards are in view, down to 50%. Zoom only changes your own view.