The width of the question box is fixed. But what's the best fitting length? Long questions make people scroll up and down. I want to try to shirk this possible inconvenience in my questions. How long can the question box be before scrolling happen?
Don't worry about how your question fits the reader's screen; worry about how it fits their brain.
Very short questions (1-2 lines) are almost universally bad (I exclude "evaluate this expression" questions). They tend to generate a stream of comments: in what sense do you use term A? What is the domain of operator B? What is the source/context of this dubious claim you find confusing? Etc.
On the other hand, long-winded questions run the risk of readers never finding out what the question actually is. Not everyone wants to read paragraphs of prose without knowing whether the question is something they like to engage with.
My suggestion: make it as long as you want, but
clearly isolate the actual question,
state it early on, and separate it from additional information (definitions, motivation, partial results)
(Formatting preferences vary; there are more than one way to achieve such effects.)