The total height of the speed graph will be given by the fastest movement you make in your animation because speed in After Effects is measured in px/sec, the faster an element moves, the more px/sec. You also have an Auto-zoom graph height button at the bottom, you can use this to adjust all the curves vertically. José Panadero and OussK have already answered clearly how to zoom in and how to zoom out, activate the zoom tool with the letter Z and by clicking simply zoom in, if what you want is to zoom out with the same zoom tool activated, press Alt in Windows or Option in Mac and ready, click and zoom out.
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