What was the size before this one? Did it have a solid root ball then?
I use 5 gallon 15 inch wide pots for smaller growing autos and photos.
Its not hard to check. Right before it needs water again and the root ball its dry just squeeze and work the sides of the pot a little. grab the stalk and lift. If it comes out in one chunk it needs replanting.
7 gallons really isn't that big.
These will give you an idea of how it is done.
Notice the yellowing in those big plants? Sometimes it is a sign they need transplanting.