The Offset is the size measured by pixels or something like that. But Scale is the ratio with the parent. For example, if you set it to 0.5, it will be half the size of the parent object. The parent in you case is simply the screen.
Edit: Once it is set to Scale, you can manually scale it and it will stay with scale. So you can keep one as a reference, set the other to a random scale, then scale it manually to the size of your reference and then change the other to the same values as well.