For an indoor grow, what size container is the big...

All depends on how many plants you plan on using in your 4x4 and how long you would want to veg for. 7 gallon is around 26L so I reckon you could squeeze 4 of them in a 4x4 and is the largest size I would consider for sure. I normally use 4x 23L for soil in a 4x4 or 4x 20L for coco. Now there are other ways for example I have just done a single 20L pot with coco and scrogged and it takes up the entire tent with around 40+ main heads although veg time was very long. In a country without plant number rules I would still go to 16 plants in 7-10L pots as the veg time would be smaller allowing more cycles per year. In Spain I even did 25 plants in 7L pots with only 7 days veg and could still pull 1-1.5 lbs (5-600g) per 4x4. Hope this helps pal👍