Someone once told me - rule of thumb - a sentence should never be longer than a certain number of words. I can't remember how many words she told me. Has anyone else ever heard this?

Not sure what number it would be, but letting it go on too long turns it into a "run-on" sentence.

If you can't say it comfortably in a single breath, it's probably too long. There isn't really a "rule" for this sort of thing, you just get a feel for it after you write for a long time.