That's about the size of my veg room and with a 400 MH I can keep day temps 77-79 most days. So depending on where you are and the climate there you may have heat issues. Being you are using only HPS I assume that will help. It gets super tight for me by week 4 of veg with 4 girls, I couldn't imagine flowering mine in there. If you just plan to do seed to harvest in 3 months that 6 feet should be enough for you depending on strains. The good and the bad with TOPPING comes into effect here. If you don't top you will have good air circulation and plenty of room to maneuver around and spot check your plants, but may run into plants growing too tall, some LSTing may be necessary. If you top in that small of room with 4 or more girls it will get hairy. So I would not TOP or FIM.
Being you are going to flower in this room to you need to stay light proof and you need fresh air so you need a intake fan that has a light proof setup, and a exhaust fan and or carbon filter combo that is rated 25-50% more in CFMs if you wish to keep the smell under control with negative pressure.
Minimum gallon for every month of growth. I'd do 4 at most at first.
If you want a decent plant you have to transplant a few times. As example I start in solo cups about 9-14 days, then into 5x5x5 square pots for about 12-20 days, then into final pot at least a week before you switch to 12/12 lighting. I use to use 4 gallon square pots but I have found fabric pots have way more pros then cons. Much better growth then standard pots. I use 5 gallons.