Pinch your fingers together or push them apart if you're using a trackpad to zoom in and out. Use the trackwheel on a mouse.
You can also zoom using the Here interface. Next to the room map in the top right corner of the room, you will see a pair of + and - buttons. Click on these buttons to zoom in and out of the room. The + button zooms in and the - zooms out. The buttons look like this: