That's tough one, I looked on the RSForms forum and they only accept support tickets for this assistance so you might try that. I can't tell you which file in RSForms you would have to edit, it looks like all the styles are inline so CSS won't do the trick here.
I googled "change recaptcha width" and found various answers, some I saw said you can't change the image size, others referred back to the link you have or offered similar solutions. It's all about finding which file has the script in it and modifying it to see what is possible.
To be perfectly honest, I think it looks fine as is. It's a little bigger than the fields but I don't think it's the end of the world. I'm not sure how all of that stuff would fit in there if you made it smaller. Maybe consider making the Description field bigger to match the Captcha size, that might even it out a bit. Just a suggestion.
The difficult we do immediately, the impossible takes a little longer.