This is Rainbow Text. Test this out for yourself.

Type your text in the box.
Select how many rainbows you wish to appear.
Click submit.
Click on the "highlight code" box.
Copy the code.
Open up a new post.
Click on the HTML box (located under the font size box on the edit tool bar in the advanced editor).
Paste the code into the HTML box.
Close the HTML box.
You should see your rainbow text.
You can highlight then change the font size.

Rainbow Text Maker

Here are a few others:

Draac -- You can choose your font type. They have other types of generators here also along with website building tutorials.

PHP Rainbow Text Generator Rainbow Code Maker -- font choice but must choose their colors.

htmate - multicolored fade text and lots of other goodies here for your website/blog. (Warning: language usage.)

Another one - at RedBirds with text options

Text Colorizer at Chami - you select your beginning color and end color. You can also choose your font and font size.

Note: If you wish to change the text size, not the font size in the HTML code and just change it or add it to your code.

Tip: I used the Text Colorizer at RedBirds then Chami.  I liked the upper and lower case letter combo offered at RedBirds however, I liked the coding at Chami.  So I used the Chami code and then just customized it like the RedBird code with the upper and lower case letters randomly.  I also used a HEX color chart to change a few of the colors I did not like in the code.

Also, why are my comment-tators fonts so large, and some are blue, when the rest are consistant size and color? tBlog burp me thinks.