Giant Man could probably not even achieve a size necessary for him to be able to withstand Thanos' blasts or match his strength. They're both smart, but Pym's smarts lay in science, and Thanos' smarts lay in kicking ****ing ass on a galactic scale.
I own at least 800 comics. Trust my ill begotten wisdom.
let me just throw my hat in there and say that i own at least 200. it not a matter of how strong he can get. even if he grows to optimum height, i dont think he would be able to stand up to concussive blasts of cosmic energy, not only that Thanos can wield disintigrating energy. so ther is no way he dtands a chance