When you select a folder of photo's it re sizes them to fit the size of gallery you are putting them in, plus a thumbnail
I am guessing you want to batch so they are all the same size
save the gallery as per normal into a folder on your desctop
Then go to that folder, inside you will find the files folder, with both sets of images in, big, small, big, small, the small ones are obvious .s
Just drag all the big ones into a new folder for your project
and you can size any photogallery theme by dragging the frame size.