I had this same problem too. Fortunately its an easy fix.
From the dashboard, go to Gallery>Manage Gallery. You should see the gallery (and any others you have created) listed, with a checkbox and ID # to the left of each gallery. Simply check the box next to the name of your gallery and select “Resize images” from the action drop down menu. If you haven’t previously specified image dimensions then you’ll be given a width and height box to fill in. Once this is complete just hit the “Apply” button to the right of the drop down and all the images in the gallery will be resized.
While resizing after the fact is easy enough, you can avoid this little chore in the future by setting NextGen to resize the images upon upload. From the dashboard go to Gallery>Options>Images. On this screen you will see a number of settings, including height and width attributes as well a box to check to “Automatically resize” images.
I hope that helps- please let me know if you have any questions.