3x4 or less. Use the JPG format. You must know what directory the Pix is in when you want to attach it to a message. The main idea is to produce a Pix that fits in your viewer window. Then it will print on one piece of paper.
Kilobytes are best for email.
Use the JPG format. Now, test it out. Open a new message window and attach the pix. Then double-click it and open it. It should fit in your viewer window.