1. use nesting in new compostion (in project window drag existing comp to comp icon, this will automaticaly adjust previouse comps setting, such as lengt etc).
2. CTRL + K to change new comp setettings. adjust your target size.
3. scale nested comp to new dimensions.
4. switch on Continuously Rasterize in timeline
somtimes this rasterization blows up scenes with nested 3d comps, but generealy it should work.