Recently I wanted to resize all the pictures I have uploaded to my blog and Google+ etc. They end up in a Picasa web album, and in the web interface I did not find a "resize" option that you could use on a lot of pictures at once. My Google skills also did not give anything useful (only people talking about how inconvenient it is that there is not batch processing option).
Anyway, I found out how to do it, at least for the size I wanted to resize my photos to, and since no one else seemed to have written anything on how to do it, here goes...
To resize all the photos in a Picasa web album without much effort do this (only works when resizing to sizes you can select as "Default upload size"):
1) Install Picasa.
2) Import your web album in your local installation of Picasa.
3) Click the option "Sync to web" to set it to "On". (Can be found in the upper right corner of the part showing your album in Picasa, and in other places too I think).
4) Edit your default upload options and set them to what you want to rescale your photos to. Tools->Options->Google+ photos->Default upload size
5) If you do not have time to wait for it to automatically sync, you can force syncing by right clicking on the Picasa folder, Online Actions->Refresh Online Status.
All your pictures downloaded from the web album will be re-uploaded with the new size.