You will see the options from left to right
Websites - 1x1 Recommended - View Grid
By clicking the Websites button, you will see more options (shown in the image below this one).
As you can see, you will be able to choose your Tiles between either:
Websites - News Feeds - Widgets - Import Browser Bookmarks
By clicking any of these Tiles or dragging them to an empty cell of the Webmix, you are editing.
You will be able to decide the size of the Tile BEFORE You add it to your Webmix.
Make sure that your Webmix's grid has the space for the Tile you wish to add.
Lastly,
Do not forget to update your changes in case you have shared or published this Webmix
Please feel free to reach out if you need extra help; we would be happy to assist you.