I know that in Times New Roman, different letters have different width sizes. What I am trying to find is if there is info on what the measurement is for the widths in font size 12 for it. i.e. for the letter "a" what does its width measure at in font 12.
Times New Roman is Microsoft's version of the originally Adobe font "Times", and was designed to be metrically equal to this (almost*). For the original Times, you can find online files of its metrics:
Times.afm. These metric files are freely available for a number of Adobe fonts, because it eases development of PDF tools ("Times" is one of a handful set of fonts that are "required" to be present in PDF reading implementations).
An AFM file is a simple plain text document that lists, among others, the exact bounding boxes and set widths for all characters. Locate the line
StartCharMetrics in the file; below this are the measurements for all supported characters. The measurements are in the design size of the font (an imaginary grid on which the coordinates are given). The standard design size for these fonts is
a, this looks like
C 97 ; WX 444 ; N a ; B 36 -9 442 460 ;
C is the character code,
WX the set width,
N the name, and
B the exact bounding box. You want the
WX value here:
444. It means that an
a drawn at 1000 pt (or any other unit, really, be it inches or mm) will be 444 pt wide. To get the width at 12 pt, simply calculate
12*444/1000 = 5.328.
Type size is generally in points; a typographer's point is 1/72 in, and so you can get a measurement in inches by dividing by 72, and in mm by multiplying this last value again by 25.4. Your
a at 12 pt should be 0.074 in/1.8796 mm wide when printed out, and in an 8′ width you can fit 8/0.074 ~ 108.1 a's.
Repeat for all of your characters to get the width of any (unkerned) text. (And if you do need to take kerning into account, those values are usually supplied in the AFM file as well.)
* Interestingly, "almost". The intention was there but Adobe's design grid is 1000, while Microsoft's is 2048. Therefore there is a round-off error possible of 0.5% in either direction – some characters may be imperceptibly narrower, others wider. It is only discernable by very careful measuring, but if it really bothers you, you will have to look up the exact values of Times New Roman.
Its width for
a, for example, is 909 units on the design grid of 2048. Converted to decimals, that would be 443.85 – 99.97% of the Times
Type text in a vector drawing program. Convert the text to outlines. In Illustrator apply Type > Create Outlines. In Inkscape apply Path > Object to Path.
Ungroup. Select an interesting character and see in the info panel the width of the object. Have a good measurement unit set in the program preferences.
The preceding method gives only the width of the glyph. But you probably need more. Letter spacing needs also room and it's complex, if balanced look and easy readability are wanted. It depends on which letters are adjacent and the wanted general style. Advanced software such as Adobe InDesign can place letters substantially better than many other popular programs, for ex. Word. The difference is substantial, no matter do both programs use the same font or not.
In font editors you can easily extract typographical measures which should be interesting if you plan to develop your own font. Know that the subject is less trivial than one can easily think.