I would let them get alot bigger than that.
look into to topping or scrog/ fim (for next time) if you have space restrictions.
If you put them into flower now it will take forever till harvest.
Some people flush every few weeks or month. I don't like to flush that often
cause youflush your nutes out as well.
I feed hevy then water hevy. I don't do a full on flush ,(filling your container with 3x it's size in water),
untill two weeks b4 harvest.