Well, if you place the light on the opposite side then it isn't a fill, it's a second and conflicting 'sun'.
But if you do use a fill, then 90+% of the time it will be directly in front of where the face is pointing, and if the face is pointing towards the camera then that is where the fill goes, it's called an on axis fill because it's on axis to the camera lens.
Different sizes of fill light will produce slightly different results, but it really makes little difference, the position is far more important than the size.
I frequently use a very large softbox, or a shoot through umbrella, directly behind me. That's because either is plenty big enough for most of the light to reach the subject, regardless of my body partially blocking it.
But it also works with an on camera flashgun or a pop up flashgun acting as fill