If we're just talking about whether to call it a novella or a novelette,, I'd hope that it's Jim. But my guess is the publisher. If we're talking about the decision to write a short piece vs. a longer one, I might say the same thing, but I don't think Jim went through his publisher for The Law, so Jim?.
I just Googled it and went to a couple of sites, basically length or number of words are important.
A novelette is longer than a short story but shorter than a novella. The word count is usually between 7,500 words to 17,500 words.
Complexity is important, usually according to the web page I visited a novella is typically from one character's point of view, that fits, one was from the point of view of Thomas, the other was from Murphy's point of view. However in latter years that has changed somewhat as far as number of "main" characters in either, also there are generally no chapters. Apparently chapters are also found only in novels. Anyway that's what I found.