There’s no such option when installing Cubase. (From the top of my head the only plugin that is somewhat separated when installing Cubase is Halion Sonic SE, so you might be able to duck that installation.)
I would think the best approach might be to do a regular install and afterwards disable the plugins you don’t want to use (with the “Plug-in information” window found under the “Devices” menu) and then delete the large resource data files used by (some of) these plugins.
I don’t think this approach will compromise stability when using Cubase but the question is what happens when the next update is released? (Probably not all that much but you might run into some issues depeding on what the update contains, e.g if there’s loopmash content updates and you deleted the intire sample folder.)
In the first installation screen there is a button with options in the lower left corner. There you can de-select what components you want to install. Excellent!