Initially all the channels in a view have equal heights. To make a channel taller, drag the gutter between it and a neighboring channel. The gutter is the horizontal empty space that separates a pair of channels. Position the mouse cursor over the gutter, so that the cursor changes to a double-pointed vertical arrow. Now press the left mouse button, and while holding it down, drag the gutter vertically until the channels have the desired heights, and then release the left mouse button.
To restore a pair of channels to equal heights, double-click the gutter between them. To maximize one channel while minimizing all others, right-click the desired channel and select Maximize Channel in the context menu; this is useful for files with more than two channels. To restore all channels to equal heights, use View/Fit Vertically (Shift+0).