There is no direct setting within Excel to change the size / height of the tab at the bottom of each worksheet.
However, there is a workaround that may or may not appeal -
To increase the size of the tabs right-click on the Desktop and select Display > Appearance.
Increase the size of your Scrollbars.
Note: this will affect all Applications, not just Excel, and will only apply to your PC (i.e. it will not be seen on another user's PC unless they make the same changes to their settings).
See click here