After you’ve added your columns, you can resize them by hovering in between the rows until you see the small arrows appear, then dragging them to the left and right, then releasing them once you are happy with the split.
Any components within the columns will automatically resize to fit your new layout. This is a good way to make sure your components are all at the size you need them.
Components will always fill the column from edge to edge, so the bigger or smaller the column, the bigger or smaller the component will scale.
Rows are a little different. Rows cannot be dragged manually because they will always adapt to the components they are holding. This means that you don’t need to worry about having big empty gaps at the bottom of your content. If you need to add more components, don’t worry, you can always add more into your row or simply add another row.