Drag an outer edge. Hover your mouse over any of the outer edges of the collage and you’ll see a thin blue bar. Drag the bar left or right, up or down, and it will resize the entire collage along that axis.
Type in the dimensions. You can also type the exact pixel dimensions you’d like to resize to (e.g. 800 x 600), in the white toolbar below your collage. The largest size is 3400 x 3400 pixels. Hit the padlock icon to lock and unlock the proportions.
To tweak the dimensions of the images within your collage:
Drag an inner border. Hover your mouse over the inner border of a photo or empty cell, and you’ll see a thin blue bar. Drag the bar to adjust the sizes of the images, and watch them shrink, grow and re-arrange themselves beautifully.
By the way, you can add new cells to any collage. Just move a photo onto any border and when you see a thin blue rectangle, and drop your photo into it.
Remember, too, that you can also change the background color, the spacing between your photos, and the roundedness of the photos by clicking the 'background' tab (the icon that looks like an artist's palette).