You will have to do this frame-by-frame. This is fundamentally how animating with Pencil2D works (with the sole exception of the camera layer). There are other animation programs which support what’s called tweening, that allows you to specify two end points of position or in this case, scale, and the program will automatically interpolate the frames inbetween the ones you defined. You can take a look at this guide for other animation programs if you really want this feature: Pencil2D Alternatives. OpenToonz and Synfig are two notable open source programs that support tweening, although be warned that such programs are generally have a much steeper learning curve than Pencil2D.