Can I resize my uploaded photos in Google+ to standard size? I'm about to run out of Google Drive space and the standard size would be enough, but I can't find an option to resize the pictures. It would take so much time to reupload them with Picasa. If I can't, then is it possible that picasa resize and upload the pictures and replace the originals?
You can but it is tedious..
I did the same thing but, luckily, I only had 1GB of space to claim back from G+ photos. I had them set to standard (<2048x) size on auto-upload but when they introduced full-size auto-uploads, it became the default setting and so a couple of my albums backed up at full size without me knowing.
To change the size of the photos, go to an album, open the first photo up in the lightbox, click 'edit' above the photo. The Creative Kit will pop up and at the bottom on the left toolbar is 'resize'. Google does help you out a little bit by presetting the resize values to the 'standard size' of <2048x along the longest edge. So it's a fairly quick process if you can be bothered. Once you've resized, click save in the top right and 'replace' when given the option. Then just click to the next photo and repeat.. As I said, it's doable but can be time-consuming.