From the looks of it you can use the GetGlyphData() function of the ID3DXFont interface. Pass in the address of a RECT and the glyph identifier (should be just the character casted ti a UINT), and the RECT should get filled with the minimum character area. Alternatively, if that dosnt work, you can get the HDC of the font and then use the GetTextExtentPoint32() WIN32 function.
HDC dc = mpDXFont->GetDC();
char charater = 'A';
GetTextExtentPoint32( dc, (char*)&character, 1, &size );
// size.cx // width
// size.cy // height
The Glyph function should be more reliable though (if it indeed does what it looks like it should be doing) because that's part of the interface.