For prototyping, you wont need any bigger than the prusa's build volume, in fact less would do. Once you get bigger than that the parts stop being very practical because the material and process isn't particularly high strength.

For model making, it depends on how big you want the model to be. One thing to note is that a mendel style printer like the CR-10, especially a big one is either going to go slow and take forever (several day long prints), or leave you with nasty ringing artifacts which may be difficult, time consuming or impossible to remove without losing the details from the model. I'm assuming surface finish is important to you for the models.