What is the ideal size to a binary CNN? Is my dataset long enough?
There is no mathematical function that is able to relate size of model and performance.

Architecture(filter size, activation, depth, weights initialization) of a neural network can be thought as hyperparameter and hence to improve your model you need to employ hyperparameter tuning methods.

The other way to improve your model is to experiment with state-of-the-art features with your architecture. I believe your dataset being small enough give you a lot of room to experiment(because of the supposedly fast training time).