The two most common dimensions we’re constrained by are space and time. If setting a time limit is timeboxing, then perhaps the space-analogous exercise can be called sizeboxing. You pick a limited size for your work and work within that.
One popular format I’ve seen is an essay that fits in a screenshot on your phone. When working on articles, I write my notes to fit a 4-by-6-inch index card; any longer and it has to be a new note. This keeps me concise.
If you’re recording music, scale down by committing to recording a song with only two instruments if you usually use more; or if you want to produce a lot of ideas, commit to writing thirty-second melodies for one week.
If you’re working with paint, choose a surface with dimensions no more than four inches by four inches.
If you’re programming, restrict yourself to a set number of lines of code or a specific memory size. (Sizecoding might be an inspiration.)
Another version of this is filling out three pages of writing in a notebook. (If you do this without stopping, that’s what teacher, artist, and author Julia Cameron calls the morning pages.)
The less time you have, the smaller a size—or the fewer the elements—that you may want to go with.