How do I know how to construct the layers of my CNN

Question

I've done a CNN project with Keras and OpenCV, and I've got roughly 65% accuracy. And now I have to present this work in my University, but I'm afraid if the teachers ask me for how do I knew how to construct the right layers to my CNN.

In fact in my development I took a look in others projects to see how the other people do, and I know that you have to have a number of neurons equivalent to the number and size of your image. But I know that has a mathematic behind this.

So how could I tell if they ask me it?