Well, for image size is very relative because it need to be adapt to any devices (responsive). But, we can recommend you this:
Fullscreen image use 1280x1024px, better 1920x1280px
For image gallery & portfolio min width is 540px, larger is better.
Also, it would be great if you compress image before you upload them. Our favorite tools is ImageOptim app, maybe you have another one 🙂