Function XPLMGetFontDimensions 

pub unsafe extern "C" fn XPLMGetFontDimensions(
    inFontID: XPLMFontID,
    outCharWidth: *mut c_int,
    outCharHeight: *mut c_int,
    outDigitsOnly: *mut c_int,
)

XPLMGetFontDimensions

This routine returns the width and height of a character in a given font. It also tells you if the font only supports numeric digits. Pass NULL if you don’t need a given field. Note that for a proportional font the width will be an arbitrary, hopefully average width.