Resizing images is handled differently depending on what editing software you are using. One mistake I have seen people make is that when they change the "VIEW size" they think it changes the picture, but it doesn't. That's just a zoom-type feature.
To shrink an image make sure you actually select an option that is a physical image resize. In some programs you can select a percentage, but you'll want to watch the pixel size. For shrinking, you should try and get the width of the image down to at least 550 pixels. Just make sure you have it set so that it will automatically adjust the height to maintain the proportions.
As for enlarging pictures, this can be problematic. If the image is small but has a high resolution, then blowing it up shouldn't be a problem. But if you have a small image with a 72 resolution, when you blow it up it's going to look bad. But to do the enlargement, you would do the same as above, but in the reverse.
HTH