No, it is currently not possible to change the photo size. The picture size is set to fit the layout of your chosen template in the best possible way.
If you're not happy with the picture size in the template you've chosen and want your picture to be bigger or smaller, don't worry! A different template might just have what you need. You can change the template. Here you can read a step-by-step guide on how to change the template after completing your resume.