Right click the task bar, select properties,
Click Start Menu tab,
Select Start Menu,
Click Customize button,
Click Small Icons.
OR
Click Classic Start Menu,
Click Customize button,
Scroll down and select Show Small Icons in the Start menu.
Another place to look,
Control panel,
Display,
Appearance tab,
Effects button,
uncheck - Use large Icons,
Click OK,
Click Advanced button,
Click Item: Desktop,
Select Icon,
You can change the size in the adjacent window.