Irfanview is a free download. I haven't done much with it, but sometimes use it for re-sizing because it's so quick to use. You can also batch re-size all images in a folder, for example.

1. File > Open, find image
2. Image > Resize/Resample
3. In Set New Size box, set the new size on only one side. Make sure the Preserve Aspect Ratio Box is checked at the lower left, click OK.

Batch processing is a little different:
1. File > Batch Conversion/Rename
2. Select the images, and what folder you want them to end up in (I sometimes make a new folder within the folder that the original images so it's easy to find them) and the rest of the controls are pretty similar, but they're in Set Advanced Options at the bottom right of the window that pops up.