My goal is to display Terms & Conditions using Modal because is the most logic method for let user correctly read my terms without hassle. Others method such New window or Iframe are suck especially if they use mobile device.

Before when I was on CB 9.1 I had installed the excellent Modal plugin from NoNumber:
www.nonumber.nl/extensions/modals

This plugin was very helpful for set my own Modal using any kind of dimension. Now CB 2.0 have is own modal Term and Condition feature which is a great point BUT the big downside is we can't configure the modal to have width/height in percentage such 90%. We are forced to set fixed dimensions and this is very bad because we must take care to Desktop VS Mobile users. Due to the new manner to set URL, we can't anymore use Modal plugin.

So could you allow us to use percentage with modal ?

well I finally found a solution. I unpublished the default CB Terms & Conditions field and I created a new multiple checkbox field. Then I have set my Url and the modal code inside the label field.

However is too bad to be obligate to use third party solution.