Quote - Will it render at one of the smaller sizes?

You could switch to custom and try eliminating subrays or lowering quality or eliminate  depth of field calculations, if it features poser imports there are many sepaerate ways to optimize that. You could try hiding once of the complex items from render and see if the scene will render then, and if so there's where you can look towards for optimization

Something I don't know how to do and I've never found any tutorial  about this problem.Do you know links that would be useful to understand what you said?

A print screan would help me to understand?

Thank you for your help