CMcInAZ

I recieved pictures of a recent holiday in an email and when i tried to save them they saved as 320 x 240 meaning i can hardly see any of the detail in the photo. I uploaded photo's of my own and they are much larger 2048 x 1536. I am wondering if there would be any way to resize the smaller pictures to get them a similar size to the others without losing any of the clarity in the picture? I have tried Image -> Resize but it just results in a big blurry mess. Any help is appreciated