Will Increasing the terrain size increase apk file size

Yes, it will increase the apk file size, if your terrain all flat it will increase a little bit(depending on it's size), Also same for a terrain with hills/bumps, but for the most part it won't take a lot of size though.

No, not by itself. Just increasing the real-world size will make the "height-squares" larger, but you still have just as many, so no extra data is stored.

Increasing the heightmap resolution(?) (the amount of adjustable grid spaces) will make it larger. Of course, if you make the terrain larger and want the same amount of control, you also have to double the resolution.