Measure the Arial font on the page using a ruler. Start at the bottom of a letter that goes below the line, such as "g," and measure to the top of a tall capital letter, such as "A."
Convert your measurement in fractions to a decimal number. For example, if the measurement is 3/8 inch, divide 3 by 8, yielding 0.375.
Multiply your decimal number by 72.27 to arrive at the number of points in the Arial font. If your measurement was 0.375, the number of points will be 27.1 points. A single point is very small, so unless your ruler is extremely accurate, round your point size to the nearest whole number for practical use.
Use the same process for calculating the points in postscript printing, substituting 72 for 72.27 points per inch. Since the formatting regimen for the fonts are not discernible on the page, your final result will be an approximation.