So far, in the MCU, we haven't seen any evidence that Hank Pym has tried to use his Pym Particles to make giant-sized people. We do know he's developed the growing technology, since he gives Scott "shrinking" and "growing" disks that, among other things produce:
A train-sized sized toy train and a dog-sized pet ant.
The only thing we haven't seen them do is make a person bigger than person-sized, so it's possible there's a limit to how big they can make things. With that apparent limitation, we're left with Scott being able to go between normal sized and ant sized using the suit.
However, it's incorrect to state that Ant-Man only has two sizes. He only uses two sizes, but the suit is capable of more. This is a key plot point in the climax of the movie. The suit contains a "size regulator" that prevents it from shrinking the user smaller than the size we normally see Scott in. When the regulator was disabled,
Scott (as well as Hope van Dyne) shrunk down much smaller than an ant, to what Hank calls "quantum level".
In other words, the small size we see Scott use in the movie was calibrated specifically by Hank to be the best "smaller" size he feels a person can safely take on. There doesn't seem to be any technical reason Scott couldn't stop somewhere in the middle. The question, though, is what would that accomplish? Is there any benefit to being the size of, say, a mouse or a cat, instead of an insect?
Though it's not shown, we can probably assume that Hank, during his time as Ant-Man, figured out by trial and error what the most effective small size was, and decided that it was "approximately ant sized." That sizes gives him the benefits of a smaller, denser frame as well as allowing him to make use of the actual living ants he can control.
Of course, thematically that also works out perfectly: the character is Ant-Man, it only makes sense that he works at two sizes: Ant and Man.