I agree. That would be great to add so that we do not have to leave the Wix editor page. You may already know of this, but this is what I use when I don’t have all devices available for testing dimensions. Screenfly / Test Your Website at Different Screen Resolutions . Hope that helps.
As Wix continues to improve, hopefully this, along with a dedicated Tablet Editor (with all dimensions as well) just like the Mobile Editor, would be an amazing addition.