At any stage, you are able to preview your donation page BOTH on desktop and mobile whilst in the editor. This is important, especially on mobile to make sure it not only meets your needs but presents well on all devices.
This mode will show the page in a wider, full-screen view. This will be similar to how your donors will see the page if they are using a laptop or a desktop PC.
Note that the preview is limited to the size of the PC you are working on! Some of your donors might use a larger screen or monitor, so test the experience on as many different devices if you can.
This mode should open up in a tall, skinny window with the same width as a phone/mobile device. This will be similar to how your donors will see the page on a mobile phone - iPhone or Android device.
Note that the tiles will "flow" automatically from a horizontal layout to a vertical "stack" to take better advantage of the space on smaller screens. Also be aware that some elements can be automatically hidden on mobile so that your donors don't have to scroll too far.