Mail.app should already do this.
If not, then you can create an script/workflow using Automator that you can use again and again for quickly resizing photos you selected to the resolution you specify. I wrote a script like that using Automator so that I can quickly change the size of a large batch of photos (or sometimes just 4-5 for posting here, or for emailing), and these photos then open up in Preview. 🙂