Once you are done with the worksheet visualizations, then go to new Dashboard. There you will see all the worksheets that you have created in Tableau, so now drag and drop the worksheets that you want to show in to the dashboard. Left side of the dashboard there are two options Dashboard and Layout, if you click on dashboard then under that you will see the Size option. Now if you select Automatic then the size of the dashboard will automatically set as per your screen resolution. But if you want to do it manually then you can select the fixed size option then custom, from here you can set the size of height and width of the dashboard.
Now if you select the fixed size and custom, set the width and height as 1000*500 px.
In this way you can resize your dashboards in tableau. if you want to learn more about tableau analytics and visualizations then you can check Tableau Certification