How to Increase the pixel size of a .jpg from Iphoto?

Steve,

iPhoto stores the photos exactly as they are when you import them. Select one inside iPhoto. Open the Info Pane (little i, lower left of the iPhoto window) and see what pixel dimensions are reported there.

You can downsize a photo when you export it, from within the Export dialog. But there is no way to add pixels that are not there. If there were a way to do it (in an image editor), the photo would lose quality.

Check the quality settings on your camera if your photos really are that small. Perhaps it has a "postcard" setting that is giving you this size. It's best to shoot in the highest quality your camera permits.

Steve:

If these photos are from your digital camera you should check the menu to set the size to the largest the camera is capable of. You can always downsize but you can't upsize.

Do you Twango?

TIP: For insurance against the iPhoto database corruption that many users have experienced I recommend making a backup copy of the Library6.iPhoto database file and keep it current. If problems crop up where iPhoto suddenly can't see any photos or thinks there are no photos in the library, replacing the working Library6.iPhoto file with the backup will often get the library back. By keeping it current I mean backup after each import and/or any serious editing or work on books, slideshows, calendars, cards, etc. That insures that if a problem pops up and you do need to replace the database file, you'll retain all those efforts. It doesn't take long to make the backup and it's good insurance.
I've written an Automator workflow application (requires Tiger), iPhoto dB File Backup, that will copy the selected Library6.iPhoto file from your iPhoto Library folder to the Pictures folder, replacing any previous version of it. You can download it at Toad's Cellar. Be sure to read the Read Me pdf file.

If you're stuck with 640x480 right now and you want physically larger images, take a look at Photoshop Elements. It has a resize feature with several options for the algorithm employed. Depending on the kind of image, this type of "enlargement" can be decent.
John

John's got a very good point there. As long as you're not trying to enlarge too much you can get a fairly decent enlargement with PSE. You can then add some unsharp mask sharpening to it. Amazon.com usually has deals on it for about $80. Also, if you happen to be in the marker for a scanner, Canon packages PSE with many of it's scanners.