Images are sized in 'pixels' (width and height). Select a width of around 500 pixels wide. Most apps or programs automatically adjust the height through 'proportionate resizing.'
Resize and compress your image by:
Using a photo editing app on your smartphone or iPad.
Uploading your image to a free web-service like TinyPNG or TinyJPG. Follow the instructions on the site and you'll be surprised at how small it will be afterwards.