Short and sweet, maybe three or four lines at most. I gave my education and what I used to do for the State, but only because they were relevant to my books. I don't know what you mean by TMI or inappropriate information, but I kept everything professional. Think of it as a job application--even though agents technically work for you, they still pick their clients so the onus is on you to make a good impression. You probably wouldn't mention on your job application that you worked your way through college as an exotic dancer, unless maybe you were applying for a job where that would be useful. Likewise, you might not mention it in your query-letter bio, unless it has something to do with your book.
I should point out that I landed one of the first (and few) agents I queried, so I'm not as much an expert at query letter writing. Entire books are written and courses are taught on the subject. What works for one agent might not work on another, so there are few hard rules. However, one thing that all agents have in common is that they are very, very busy. In addition to an ocean of submissions to sift through, they also have the writers they already represent to look after. So, the less of their time you occupy, the better an impression you'll make.