What version of Captivate are you using? I have tried this with Captivate 6 and there was no limit that I could see on the variable - I entered 30 characters and they all displayed on the certificate. I don't think variables are limited, but the text caption size may cut off some characters if it's not big enough on the screen - this shouldn't be an issue with the widget as it seems to auto-size to the variable.
The source files for widgets are found at C:\Program Files\Adobe\Adobe Captivate 6 (32 Bit)\Gallery\Widgets\Source\ - you'll find the .fla for the certificate there.