You can resize images and photos as well as export them! How they export though, works differently.
Simplified only allows exporting multiple artboards in video format so long as all of your artboards are the same size.
If your artboards are different sizes, they can't be exported in video format.
TIP: When creating a video composed of several artboards, they should all have the same dimensions. You can't download a video if the artboards aren't the same size.
You can export artboards in different sizes as images only. Just select all artboards you want, and click Export as PNG.