As other has already said:
1. Open photo in preview
2. Optional: crop it to your liking first, just click in the image and draw a rectangle and adjust it
3. Change the size in the tools menu, adjust the largest value to 1000, the width and height are coupled by default
4. Go to file and export your image as jpg, adjust size with the slider to about 100k
The reason i choose 1000 pixels as maximum width or height and a file size of 100k is because this is usually good enough. You have the absolute maximum values in Steve's post just above mine.
Oh, I usually take pictures not so close to avoid the fish eye effect in cheaper cameras and then crop it. The resolution will be good enough anyway.