The size in Marks Card is used to change the size of Marks in the view. You can use it to change the size of Bars in Bar chart, Lines in Line Chart, Dot in Scatter plot etc.
Let’s create a line chart. Drag and drop Order Date to columns and Profit to Rows.
At this moment the line looks little bit thin. But let’s say you want to make it little bit thicker. To do that you can use the size. Click on size and use the slider to increase the thickness of the line.
Similarly, Let’s say you created a Bar chart. Drag and Drop Category to Columns and Sales to Rows.
The width of the bars at this moment is thin. We can make it thicker by using the Size Option In Marks Card. Again click on size and use the slider to increase the width of the Bars.