It depends on what gallery you are using. Some of the galleries are designed to a fixed image size and the use cropped images to ensure they are all the same size. The galleries do that by default.
If you want to change it, open up the index.php for the gallery you are using on line 11 or so there will be a gallery URL string that is built up. Look for something like 999c in that url. You can change the size to match another image size that you have defined in the common/properties.php file.
There are quite a few other threads that have specific code on how to change the image size in a gallery.