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I think the best thing you can do is think of WHY something has to be longer. Think in terms of Beethoven's economy of material, how his sonatas are mostly the same stuff repeated over and over in different ways, joined by a loose arc structure. In other words, you can make something as long as you want if you just keep repeating things in different ways. Think of how Fugues work, which is kind of a similar idea. The point is, "form" is a really complicated topic altogether and Schoenberg's entire point with his system and so on was to allow for the creation of new complex forms (like a new Sonata-type form?) using his different tonal material. It's not the form that gives something its length, it's how long you want to take with developing your own ideas. That's why even if there are many pieces that are "sonatas" and adhere to the form, that says nothing of the length itself. A good example is Symphonies, which really are "Sonatas for orchestra," they are often much longer than piano sonatas, but they have the same structural ideas. That's because the orchestra can be used to develop things as well as just the actual notes being played. Instrumentation plays a big role in structure there, hence usually longer works.