Try using GetTextExtentPoint32. That uses the current font for the given device context to measure the width and height of the rendered string in logical units. For the default mapping mode, MM_TEXT, 1 logical unit is 1 pixel.
However, if you've changed the mapping mode for the current device context, a logical unit may not be the same as a pixel. You can read about the different mapping modes on MSDN. With the mapping mode, you can convert the dimensions returned to you by GetTextExtentPoint32 to pixels.
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