It’s all you need to clearly and succinctly express a single idea in writing.
Yes, I made it up, but there’s some method to it.
It’s 2021, and good ideas still deserve to get printed. As a rule of thumb, a single-spaced page can fit 500 words. 250 words would leave half of that page free for helpful stuff like charts, graphs, images, illustrations, or white space (which all can be worth an additional thousand words by themselves).
Even if you don’t plan to go to print, 250 words still makes sense because it happens to be the maximum length allowed for scientific papers' abstracts.
For reference, this piece is only 123 words long, and I'm already starting to lose you.