Simply adjust the size of photos, videos or other elements with drag and drop. Put less in one row to make them bigger, or more in a row to have them appear smaller. Open the 'edit and enrich'-mode by clicking on the pencil icon top right. Then drag your pictures, videos and other elements around to create the composition you like.
Another option is to choose a different template that shows all elements larger. For instance 'big' or 'showcase'.
You can also adjust the shape of your pictures, videos and enrichments.