You can edit the featured photos plugin accordingly in the plugin manager where it lists the plugins you will see the plugin with that title on the hook location of forumhome_complete. Under that plugin at top you will see the configuration options for that plugin as far as number of photos and how many rows to show.
If you want bigger thumbnails on your site then you would increase the thumbnail size in the upload options in the admin of your gallery. You can not have bigger thumbnails one area and smaller somewhere else. The thumbnail size is the thumbnail size the program creates. I responded in your other thread where you where talking about category avatars and I explained how they are half the size of a thumbnail. Your overall thumbnails though are the size you set. Your default thumbnail size is 100 pixel threshold. If you want 200 pixel thumbnails then you set that.
Thumbnails are not square they are a processed sized image based on the threshold you set. Thus an image that is 800x600 pixels would be with a 200 pixel threshold a 200x150 pixel thumbnail. You must maintain the aspect ratio of an image.
There is a feature if your using Imagemagick to make uniform thumbnails which are a crop of the center of a photo. That is a crop and not a true thumbnail of the entire image.
Basically any image that is processed on your server using the server image processors has to maintain the aspect ratio of the image otherwise you get a distorted image which is what you saw when you tried to incorrectly modify your code on the category avatars to make them a size larger than the actual thumbnail is.
You can make sure your processor is set to imagemagick as it was not and under upload options set the uniform thumbnail options.
I did it for you and as you can see they are center crops. Personally I do not like them never have but your choice. Also category avatar is not compatible with that so I turned that off. You can turn that on in index options if you go back to regular thumbnails.