They are all part of the same metrics. Font size is given as either the height (in points) of the bounding rectangle of each character in the font, or the maximum height of each character when parts of the character can extend above or below this rectangle; or the width, or some combination of features. The width of each character in the font will be proportional to the height as set by the font designer. In fixed width fonts each character has the same width, in proportional fonts the width is as much as necessary to display the character (cf. 'i' and 'w'). Once you have selected a font into a device context you can get the text metrics which includes height, width etc.
The following code gives a sample:
HFONT hFont, hOldFont;
currentY = MulDiv(10, GetDeviceCaps(hDC, LOGPIXELSY), 72);
hFont = CreateFont(-currentY, 0, 0, 0, FW_NORMAL, 0, 0, 0, 0, 0, 0, 0, 0, L"Courier New");
hOldFont = SelectFont(hDC, hFont);
Take a look at the MSDN pages
] for each of the above functions for more detail.