Hal only accepts / deals with 3 user sentences. The user should then be able to keep track of how many sentences they have typed to Hal.
That's not really a good way to interact with Hal (nor real people) as usually one sentence can be properly accounted for and processed at a time.
As far as length, according to The Oxford Guide to Plain English, and other sources, the average word length of a typed / written sentence is 15 to 20 words. Any longer, people hesitate to read because it is a winding, lengthy sentence and they'll likely lose some meaning while trying to read and contain it all.
Don't forget things like lengthy words or unusual words can also have a negative effect on the reader.
While Hal shouldn't really care about these on a "bot level", it should remember that humans will be on the receiving end of its responses.
A look at some of the Tables in Hals brain can often show which words are actually being used by Hal. It is interesting and maybe something Robert or someone more up to speed with Hal's processing could explain. Then again, we haven't heard much from Robert lately.
