If you want to resize the pie chart in Tableau, for that first we will create a pie chart by using the default data set of Tableau (Sample Superstore data set). So we will try to visualize Sales across different regions present in the data. Pie chart visualization can use 1 or more Dimension fields while it can use only 1 or 2 measure fields. Double click on Regions and Sales one by one then you will see tabular visualization as shown below.
Then click on 'Show Me' on the top right corner then choose pie chart from there.
Then go to size option under Marks card, and increase the sliding bar option for resize the pie chart.
And if you want to see this in focus mode then choose Entire view, initially Standard is selected by default so you can change that. now you are done with all the changes and your final pie chart will look like below visualization.
If you want to learn more about Tableau visualizations and analytics then you can check Tableau training.