My brownies are flat I have followed the following recipe for my brownies:
Ingredients:

1/2 cup sunflower oil
3/4 cups sugar
2 eggs
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&quot; x 9&quot; 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.