I'd go much bigger than 11px... I think it's pretty much a given that the download trial link is way too small. It needs to be clear for people that they can give it a try without having to fork out $500 immediately. I don't think you need A/B tests to prove that - it's pretty much a given that 10px is too small.

I'd also agree with @patio11 - the website is outdated, unprofessional and I wouldn't trust it. I'd pay a decent web designer a couple of thousand to get a bespoke design that will be much more suitable than what you have now.