After you spawn the pill and right click on it there will be a option called [Power Meter or something else] There will be options on how strong the human will be after they take the pill
based off viltrumites

The levels from weakest to strongest.
Mark.
[Normal Viltrumite Human]
Anissa
General Kregg
Emperor Thragg

After selecting the viltrumite you want and feed it to the human they will be as strong as the people listed above.