The exact size Todd had his tool enhanced to is never stated, but the large tent he's pitching when the alien psychically awakens him suggests he's 10 inches a minimum.
The aliens plan to absorb orgasmic energy for power. Which is why the Queen is seducing humans.
The queen commanded Todd to spread his essence, but he doesn't assault any women and outright refuses sex with his wife; despite the fact she had been altered to be the perfect willing partner for him.