Unlock the taskbar.. place your mouse pointer over the top edge of the bar
and the pointer turns into a N/S compass pointer.. left click and hold, then
drag the taskbar edge down until you are left with a single height bar
again.. re-lock the taskbar..
The task bar is resizable, like a window. Put the cursor on the edge and
make it turn into a double-headed arrow. Then click and drag it to the size
you want.
If it won't resize, it may be locked. In that case, right click on it and
uncheck "Lock the taskbar," Then try again.
Also note that if you have toolbars on the task bar on multiple lines, you
won't be able to reduce the taskbar size to fewer than that number of lines.
Put all the toolbars on one line first.