Probably no more than 200 words. Anything more is not going to keep anyone interested, unless they are hardcore fans and then you can sell them your autobiography after you die.
Focus more on your regular gigs than your one-offs if you can. Making the point that you get repeat gigs at good venues will let the booker know that you can please a crowd.
Do not list each members previous bands, or history - sell yourself as a band. If you are new together, just say:
"with many years of experience between them,....lala"
And, just my opinion, but a good live shot beats a posed promo photo hands down.
YMMV