If the number 604 is stuck in your head as the maximum size of an image that Facebook will display, it’s time to reprogram your brain (and maybe your Photoshop actions/Lightroom presets for Facebook, too). The Inside Facebook blog reports that the maximum photo dimension on Facebook is going up to 720 pixels on a side, but the maximum size of profile pictures is going down to 180 x 540 pixels.
This feature is being rolled out gradually, so you might not see it right away.
If you already uploaded photos and want them bigger, those probably won’t change. Since Facebook has no way that I know of to replace photos, the only way to make photos in an existing gallery bigger is probably to trash all the existing photos and upload them again, losing existing comments, Likes, and (probably) links in the process.
Update, 2011: Facebook has increased the displayed image size once again; images can now be displayed up to 960 pixels on a side. Note that if you upload an image larger than this, visitors can click a Facebook page’s Download link to retrieve the image at the full original size you uploaded. Images you upload should always be no bigger than the size at which you’re comfortable having visitors download them. For example, if you don’t want your images to be out in the world at a size bigger than 800 pixels on a side, make sure you limit the image size of the pictures to 800px on a side or smaller before you upload them to Facebook.