Today Google+ rolled out a few updates to profiles and pages. One in particular has to do with profile cover photos; they are now 16:9 aspect ratio and must be at least 480 x 270 pixels (up to 2120 x 1192 pixels). This creates a cover photo that takes up your entire screen. Some users find this to be excessive while other people like it. When uploading a new cover photo you are unable to alter its size from the Google+ profile interface.
Following the below workaround will help you change the size of your cover photo.
1) Go to your profile, click on "change cover", upload a new photo, then click "save". You will need to use an image that meets the minimum pixel requirements. (See note at end of post regarding cropping.)
Your profile will now have the large cover photo.
2) Go to your photos, view albums, and select "Scrapbook photos". Click on your newly uploaded cover photo then "edit photo" at top left. You will be at the below screen. (Update: Instead of going to your photo albums, you can simply click on your cover photo from your profile, then click edit photo. Thanks for the tip Ray Hiltz!) If you prefer to completely remove the cover page click the delete/trash can icon instead of edit.
3) Select "crop" in the left panel then adjust the crop lines. (See note at end of post regarding cropping.)
4) Click save at top right and replace the original photo.
5) Apply the crop.
6) Your edited cover photo is complete. Go to, or refresh, your profile to see the update. You can view mine to see it live.
Note: Cropping in step one and three affects the finished image. Depending on your desired result, and the source image size and ratio, it may take a couple attempts to get it exactly how you want it. I found the best and most predictable results happened when the full image width was used in each crop.