Firstly let's start with the last question. It is normally not possible to increase the size of an image, that is, increasing its resolution without using AI solutions. But one famous solution to overcome this problem is padding the image to increase its size. But depending on the filter size, it may poorly perform if the original image is too small compared to the original input size, and filter dimensions are big. Also, in any case, if a model designed to accept the high resolution, would not perform similarly with small images because it will not get the same size of input or information (padding will just contain dummy data).
However, you can build a model that can accept different sizes, although the one that you have built will not. Your model is configured to receive 32x32x3, that is, 3072 input nodes. Thus, you have to resize all of your high-resolution images to 32x32x3 and pad the smaller images.