To Change the Size of the Thumbnail Images on display

Move the mouse near the bottom right corner of an image on the grid. Note the change in appearance of the cursor. Click and drag to change the size of all the images displayed; all the images will have the same dimensions..

Moving the mouse near the bottom or the right-hand edge of a thumbnail reveals other resize cursors that let you alter only the height or the width of the cells.

Arrows
 
If you have a lot of memory-intensive images, resizing can take a while.

 
Pressing the Option key as you drag restricts the proportions of the cells to 1:1, 2:3, or 3:2 aspect ratio to fit various screen sizes.

Drag to Resize