Measuring the font size in an image can be tricky. It isn't like looking at the font size of text that you're typing, where you can just look up at the "Properties" toolbar and see what font size you're using. The font is a permanent part of the image, so there is no way to separate the text from the background, copy it into a word processing utility and look at what size the lettering is. You can, however, use a word processing utility and a little bit of trial and error to pinpoint the size of the font.
Open the image file you're working with. Simply double click on it to open it in the default image viewing program for your operating system. Put it in "Windowed" mode by pressing the "Restore Down" button in the upper right corner of the window on the screen.
Open a word processing program, like Microsoft Works.
Type the text from the image into the word processing document.
Highlight the text with your mouse.
Click on the "Font" drop down menu and attempt to find a font that matches the font used in the image as closely as possible. This will be helpful in comparing the font size of the two blocks of text.
Use the "Font Size" drop down menu to gradually increase or decrease the font size of the text you've typed into your word processor. Do this until the size closely resembles the size used in the image. Keep referring back to the image that is open on screen to get as accurate as possible. Once the text in the word processor looks like the text in the image, simply look at the font size that you're using in the word processor and that will be the font size that was used to create the image.