In Picassa, you can edit the picture size by changing the /s---/ portion of the address to /s640/ or /s800/

Example
http://lh6.ggpht.com/_iqV4K0XxY4o/R2VWCJoQn1I/AAAAAAAABHU/Ol_LunV-4Ek/s640/Las%20Vegas%20018.jpg

will display a pic at 640 wide.


If I change the s640 to s800, the pic will appear 800 wide, as shown below.
http://lh6.ggpht.com/_iqV4K0XxY4o/R2VWCJoQn1I/AAAAAAAABHU/Ol_LunV-4Ek/s800/Las%20Vegas%20018.jpg

Like John said, over 800 is overkill, and may blow out the margins causing the page to load weird.

Both 640 & 800 will result in a photo with a correct aspect ratio, that is to say the photo won't distort like a bad
letterbox movie as seen through a knoggin full of rancid peyote and stale sudafed.