All the elements in the gallery can be resized by dragging the edges. The thumbnail strip can only be resized horizontally by dragging the edges; any resizing for height is done under the Layout tab.
To make the gallery and its elements larger, first resize the background by either dragging the edges to the larger size or using the height and width properties under the Layout tab. This will make it easier to resize each element to fit the larger size. To make the gallery smaller, work in the opposite direction by resizing the preview window and thumbnail strip, then resize the background using the options under the Layout tab. If the background is resized first, then the other elements will be cut off and cannot be resized.