Matching texture is definitely an art form. For an area that size, I would probably get 2 or 3 of the spray cans, the kind with an adjustable nozzle or various size "straws". Spray cans are expensive, but save the hassle of getting an air sprayer. I have tried the hand-pump applicators, and personally was not very good at it. You might have better success, because I do not have an artistic flair.
The first thing is to get a scrap 1/2 sheet of drywall out in the garage and practice you technique. Once you feel ready, go inside. Do plan to overlap into the existing finish lightly to get the "feathering" which will hide the boundary between old an new. Do not have any straight lines...work in curves.
Once it's done, let it be. When dry, paint the entire wall, and this will further hide the boundary between new and old.