I have seen this happen a couple times too, and the regular steps of getting it back, don't seem to work.
If you are not aware of the regular steps:
1) unlock the Taskbar by right clicking a blank area in the Taskbar area and un-check the [Lock the Taskbar] selection.
2) more the cursor to a top edge of the Taskbar, and you will see a double-headed up/down arrow.
3) drag the arrow up, to first make it thicker (or wider) and then back down to the width you really want.
4) assuming you get what you want, then just lock the Taskbar again.
Note: it helps, sometimes, to not have any applications running as selections at the time. I have seen some applications "insist" in not permitting the Taskbar width get back to "normal"
What have I done when the above hasn't worked?
Rebooted. That seems to have fixed the times it was really weird.
On a new Dell running XP,my taskbar changed to double size and I can not reduce it to one line by dragging or anything else. Any suggestions ? Thanks