I don't think the square-cube law would be an issue at all for Superman with Pym Particles, seeing as he's pretty much able to handle any weight. Hell its barely even an issue for Pym since despite being peak human at best his body is able to easily withstand the stresses of being Godzilla sized, and as we know from reality even people merely 8 foot tall or so have bone and joint issues from their own size. so I could certainly see Superman grow as big as a planet or larger.
But yeah, Galactus would be outright ko'd by a Superman that big, especially considering Thor hurt him not too long ago under Fraction.