How did Ant-Man survive the blast from Thanos’ ship in Avengers: Endgame?

Because being small can, in a sense, protect against very large impacts and forces.

If an object, e.g. a cube of side r, increases in size, its surface area increases by a factor of r² and its volume by a factor of r³.

This means that a large object has a smaller surface area relative to its volume (and therefore also to its weight), in proportion to a smaller object.

A falling ant has a mass many times smaller than that of an elephant, and the force applied to it can be distributed over a proportionately larger area at the moment of impact.

A much smaller force distributed over a very large surface area turns out to be the reason why an insect remains alive even when thrown from very great heights in comparison with its size. In contrast, very large animals would die for diametrically opposite reasons.

This explains why Ant-Man shrank as soon as he realized the arrival of Thanos’ missile and shockwave. With a smaller size, he can withstand very large forces much more effectively.