Ant-Man's shrinking disks, probably
I think it's reasonable to think he still had some discs on him when he left the Quantum Realm.
In various scenes of Ant-Man, you can see that his shrinking/enbiggening discs are stored in pouches on his hips, for instance when he grabs the enbiggening disc to get out of the Quantum Realm:
In the post-credits scene of Ant-Man and the Wasp, Scott also has these pouches equipped when he goes to gather healing quantum particles:
Then, he will still have them when he comes out in Avengers: Endgame. True, there's no evidence these pouches actually contained them, but it's not a strecth to think it, either:
- the discs are small, easy to carry around. Why not take some?
- one of these discs, albeit a blue one (shrinking's red), saved his life once. Again, why not take some?