If you can not resize the Windows Taskbar
Normally you can drag the upper part / line of the Windows taskbar and increase or decrease the height. However, some times this can be impossible, i.e., the taskbar remains as it is, it won’t let you change its height. This may happen when you have it locked (right click on any empty space of the bar and see if the option “Lock the Taskbar” is checked). It can also happen when you use the Language Bar or other toolbars. Try to close any other toolbar that uses taskbar space, and then see if the taskbar can be resized. I assume, of course, that you already have it unlocked.
Now that you managed to resize your taskbar, there is another problem you may face. After you reboot your PC or restart Windows you may find out that Windows did not save the size of your taskbar, although you had locked it before the restart. If you unlock it, resize it again, then lock and reboot, the problem remains – your taskbar ignores your preferences!
What worked for me, strange as it may seem, is not to lock the taskbar. If I lock it, in the next Windows session it doubles its size; if I leave it unlocked, it remains as I want it – just a single row.
I don’t like the taskbar being unlocked, because it has a rather thick and ugly line on the upper side (the line you drag in order to resize it), but even this is better than having a double-sized taskbar.