the tub holds about 400 gallons and seats about 6 people.
I have my deck about one foot off the ground sitting on 2x8 decking boards with 2x6 joists every 12 inches. The lawn falls downhill a bit so the lower end is sitting on cinder blocks. I actually do not have any beams.
This deck is about 8 years old and you can sit a level on it and it has not deviated from center bubble.
The key is closing up the joists and using 2 by material.