My area is a 4x4 and I have (6)- 10 quart pots (2 1/2 gallon) under a 600w. These accommodate nicely and seem to give the plants their full growth potential, but may be a bit excessive in size for an auto. If you went with a smaller 6-7 quart pot, you could probably do 10 plants in the same area without issue, but for sure in a 5x5.
One other thing to take into consideration in this endeavor would be your choice of reflector. I use a Radiant 6 and it projects light over the area well enough to cover the plants (since they are in 2 rows of 3), but would not be ideal if there were longer rows, more rows, or if I had a smaller hood. I would suggest maybe doing 8 plants rather than 10, in 2 rows of 4 to get better light coverage, but that is arbitrary depending on the reflector you choose.
I hope this helps you a little bit and if I missed anything or can add anything just let me know; I'll be glad to offer what I can. Take it easy and best of luck.