Yes it's big enough. I grew for years in three gallon pots. I would put them into flowering when they were 12-18" tall. They'd usually max out at 30-36" tall in flowering. You can transplant in flowering if necessary- especially if it is early flowering. (The earlier the better).
If you're growing autos it's best to avoid transplanting altogether.
It depends how much more you think it's going to grow. It's probably slowing down soon but it depends on the strain. Do you know the strain or otherwise could you post a picture?
I don't think you need to worry about it suffering from being root bound at that size. In fact it's about the perfect size for the pot. But you will have to water it more often. I have my grow set up so I can be away for 5 or 6 days at a time if I need to- so I would probably have that plant in about 4 gallons for that reason. A plant like yours may need watering every 3 days or so.
Basically - it's totally fine (in my opinion) as long as you don't mind watering more often. In some ways it's beneficial having more waterings like that- makes for a cleaner growing medium since it gets rinsed out better. And in flowering you really only want to transplant if it's necessary.
So - leave it alone unless it looks like it's going to get 5' tall or more, and unless you can't be around to water it fairly often.
If it's a sativa strain (long skinny leaf blades) it will tend to grow taller and stretch for a longer period during flowering. Indica strains (stubbier looking leaves) will generally have finished stretching after three weeks into flowering.
:Thumb:
Looks to me like it's probably almost done growing taller. Part of the reason it grew tall is because it was stretching for the light. If you can get some more light on it and if possible around the sides it will produce more bud for you.
It depends what kind of light you're using. Best to google whichever type you're using to find the right distance. Cfls can be up to an I sh way and some LEDs need to be 24" or they'll cause burning.
I messed up and grew to full term in a one gallon container. It got about 24" tall and 3 plants produced about 3 ounces dry.
That being said, I now go from clone rock wool, to a one gallon. Then about a week before I change to 12/12 I transplant into a 5 gallon. Waiting a week allows the plant time to acclamate before changing to flowering.
With this method I produced close to a pound with 3 plants, in a tent, with LED lights.