I have followed the following recipe for my brownies:
1/2 cup sunflower oil
3/4 cups sugar
2 tsp vanilla extract
1/2 cup flour
1/3 cup unsweetened cocoa powder
1/4 tsp baking soda
Oreos (about 15)
However, the batter didn't seem enough for my 9" x 9" brownie tin. What can I add to make the batter consistency cover my entire tin? Because at the moment my brownies have become really flat and have not risen. What am I doing wrong?
Brownies are usually not supposed to rise. If your recipe calls for beating the eggs then the eggs may provide a tiny bit of leavening, but the final result will still be about as thick as what you put into the oven. Brownies are typically a dense cake and therefore shouldn't rise very much.
Brownie batter is typically very thick, so you may need to spread it out using a spatula before baking. If there's not enough batter to cover the entire tin when you try to spread it out, then you are using a tin that's too big. Not all brownie recipes are made for a standard brownie tin. You need to check the notes for the recipe and make sure you're using a pan with the same dimensions (or at least same surface area) as the recipe states.
There are two main types of brownies: fudge and cake. Your recipe is for a fudge-type brownie.
Fudge brownies are meant to be dense, somewhat similar in texture to the confection fudge. They are often unleavened, and don't have very much flour. They don't rise much, if at all.
Cake brownies are a lighter, fluffier texture, more similar to cake. They have leavening and a higher proportion of flour. The leavening is often baking powder and/or baking soda, but occasionally the leavening is provided by beating air into the eggs.
These categories are rather fluid, and you can find recipes that exist somewhere in between the two extremes of "cake" and "fudge." Everyone has their own preference about what is "best" in a brownie. It sounds like you want (or expect) a more cake-like brownie. That should be easy to find, just search for "cake brownie recipe." Here's one from Hershey's. Some brownie recipes (like this one) and brownie mixes (like Betty Crocker fudge brownie mix) will have instructions for altering the preparation to make the brownies more fudge-like or more cake-like.
That recipe seems designed to produce a dense, flat brownie. However, you should be able to increase the baking powder (up to about 1 tsp, but 2/3 tsp would probably be better) to get more loft. Past that you’d have to use more flour and less cocoa and at that point it would be more practical to find a different recipe.