Hi Steve,
If you're a Mac user ……….
After you've imported your photos from your camera to your computer into a photo editor (I've got iPhoto since I'm on a Mac computer) you can then export them to your desktop.
When you export select medium (as I've done with your photos).
You can use the posting Preview button

- Screenshot 2016-01-26 07.29.27.png (9.75 KiB) Viewed 1206 times
to check the size before Submitting.
Your camera may have different resolution settings that allow you to alter the picture size on my Canon A700 I can alter from S (small) to L (large)
Here's a photo on S setting

- IMG_6940.jpg (10.85 KiB) Viewed 1206 times