BTW, I notice that if you are viewing an album page in your Picasa Web Albums, a size parameter (like the /s640 I inserted) is already in the address you get if you right click on one and use the Copy Image Location feature in Firefox. It's just pretty small (so, you'd want to change it to something larger like s640 for 640 pixels wide).
For example, go to this page in your albums:
Then, right click on the desired image and use the same "Copy Image Location" feature like this:
If you paste that copied link into the entry screen that pops up when you use the
button, you'd see this
Then, all you'd need to do is change the s144 to a different size (s640, etc.).