From RX's main window, you can zoom in horizontally, and also vertically on both the waveform and spectrogram views.
Zoom Sliders
The slider controls to the lower right of the spectrogram can be adjusted to control zoom. The vertical slider has two switches, one for the waveform and one for the spectrogram - their ranges can be controlled independently.
Dragging rulers and using the Mouse Wheel
The rulers to the right and below the Spectrogram display can clicked and dragged to reposition the spectrogram and waveform to show a different time range, amplitude range or frequency range.
In addition the range shown can be adjusted with your Mouse Wheel, just click on a ruler and the mouse wheel will adjust the zoom for that ruler.
To reset any of the rulers to the default display range, double click on the ruler.
Zoom Buttons
The zoom buttons allow you to zoom in or out horizontally, zoom all the way out, zoom to the current selection, or zoom to the left or right edge of the current selection.