By dragging a line of the frame (border), you can change the ratio (%) of the sizes of each frame.
Hint Specifying the size of a frame in pixels
If you want to specify the size of a frame in units of pixels and fix its size, right-click the desired frame and select [Frame Attributes]. Then, on the tab, select [Specify with pixels] and input the desired pixel value (for example, 100) in [Size].
You can fix the size of one or two frames. You should fix the size of the first frame when you want to use the rest of the area as described above. Next, right-click the other frame and select [Frame Attributes]. Subsequently, on the [Frame Attributes] tab, you can select [Specify with *] in the box to the left of [Size].