Are you talking about the size of the original files? Or the size you set in Settings > Media?
If it’s the originals, you need to upload bigger images. Trying to make an image bigger means it will be very low quality, because where do the extra pixels come from (pixel doubling is usually used). There is a plugin called Enable Media Replace, or you can simply FTP your bigger images over the top of the old ones and then use Regenerate Thumbnails to fix all the other sizes of each image.
If it’s the Settings > Media size you want to change (your originals are bigger than 200 x 200), then simply change the settings and run Regenerate Thumbnails to fix all the images.
