My recommendation is for 80 or at least no more than 120. I don't have a very big screen at work and I use split screens in vim so it's a technical limitation there. Moreover most style guides recommend 80:

https://github.com/bbatsov/ruby-style-guide
https://github.com/styleguide/ruby
http://www.caliban.org/ruby/rubyguide.shtml
http://davetron5000.github.io/ruby-style/

I couldn't find a single ruby style guide that didn't recommend 80.