How do I know how to construct the layers of my CNN 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?
There isn't a formula behind it - layer sizes are selected with a lot of trial and error (often automated by cross-validation). But saying "I've used an architecture from a network from a similar problem, or at least from the same domain" is a legitimate answer.