My brownies are flat
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.