Some email programs, like Mac’s Mail, will auto-shrink images (based on prefs) for email purposes.
Some email programs, like Microsoft Outlook, will convert your nice/tiny JPG into a huge honkin’ BMP if you attempt any in-email edits. So watch out.
Picasa is a nice freeware that has some rudimentary editing tools. And it has an “Email” button that will auto-shrink the selected photo and attach it to a new email for you.
Like @benseven says, if you have a lot of photos to share, consider an online storage instead of email attachments. Snapfish and its ilk will host full-size, printable copies for you nigh indefinitely. And your friends/family can view/print them as well.