Google Plus Cover Size 480x270 - November 2013
Google Plus has changed the cover image layout and cover image yet again. Now, the minimum is 480x270 px. Depending on how you formatted that last image size change, which was ridiculously large, it may look ok. However, you won't know until you log in an check yourself. The new image size is a minimum of 480x270 px. As you can see there is plenty of room to make it larger, just keep the aspect ratio the same.