How to prepare the varied size input in CNN prediction
Conventionally, when dealing with images of different sizes in CNN(which happens very often in real world problems), we resize the images to the size of the smallest images with the help of any image manipulation library (OpenCV, PIL etc) or some times, pad the images of unequal size to desired size. Resizing the image is simpler and is used most often. 

As mentioned by Media in the above answer, it is not possible to directly use images of different sizes. It is because when you define a CNN architecture, you plan as to how many layers you should have depending on the input size. Without having a fixed input shape, you cannot define architecture of your model. It is therefore necessary to convert all your images to same size.