To edit the size or alignment of your Content Blocks individually, go to the Design Tab and click on one of them. A menu will immediately pop-up on the right side of your screen.
You'll find here the option of adding or editing a caption. As well as many different ways to crop your image in different shapes.
You can also Drag & Drop your images within the slide. Use the grid to align and position your Content Blocks when dragging them. The grid will only be visible when moving them around.