can I change font size of checkbox caption based on button click?

This is a LiveCycle form question and should be asked in that forum.

But the answer is Yes, it's just a matter of getting the correct parameter.

Since you are using JavaScript, you'll need to check the checkbox value with: cb2-3RetainTaxRef.rawValue

However, I don't think any of your code will work because there is punctuation in your field name. This is an extremely bad practice. Remove all instances of it.

Also to see how your script is failing, check the console window. It should display at least one error related to your code.

Here's a video tutorial on using the Console. It's a little old, but still valid.

The Acrobat JavaScript Console Window - YouTube