Right click on the title block and select edit sheet format. Double click on what you want to change and change the font size.
Welcome to the forum. After doing what Christopher suggested if you close the edit sheet format function (after making your changes, right-click again and choose "Edit sheet" from the drop-down) and go to File > save sheet format you can save this change to your sheet format for the future. If this sheet format is used in your drawing template it will not automatically update when you start a new drawing until you open a new drawing, right-click, choose "Properties...", and click the Reload button so that your sheet will reflect the change. Then Save as..., and choose Drawing Template (.drwdot) from the Save as type: drop-down to save the updated drawing template.