After you insert the Gallery in your post, you will need to edit the Gallery shortcode in the HTML Editor to include the following:
This will create a Gallery with one column and one image per row. “Full size” in this case doesn’t mean the size of the image you uploaded, but the maximum width possible for the theme you are using. If image quality is important to you, resize your images before you upload them (saves on space, too).
If you prefer to have just the thumbnail image in the posting area, then use: