Continuity mistake: When Cross is seen inside Hank's home, he is wearing black gloves, a few moments later, in the same scene you can see that he is not wearing them anymore.
Continuity mistake: When Scott uses the suit for the first time, he is on a tube with a small red towel on it which disappears between shots. Not only this - the amount of food on the plate reduces and then increases during the scene.
Continuity mistake: When Scott wakes up after falling off Anthony, we can see the floor around the bed and there are no ants there. He then looks down and sees the bed is surrounded by ants.
Continuity mistake: When Ant-Man steps into the tub, he is next to a curtain. Shot swaps to a wide angle and the curtain is gone, only to reappear next to him a shot later.
Trivia: Garrett Morris makes a cameo as the man in the car when Scott hits its roof. Morris was the first person to portray Ant-Man in a 1979 sketch for Saturday Night Live called Superhero Party.
Chosen answer: The suit was built human-size and then shrunk using Pym particles. The same technology can also be used to grow objects. Cross was shown to have a hand-held device earlier in the film capable of shrinking, so he either used this device, or activated the suit's built-in growing/shrinking technology.