I think that SSD storage needs are much easier to predict than RAM usage, unless buying one's first computer ever. Just based on your current needs and whether you expect them to grow, it should not be difficult to extrapolate what will be needed.
Also, without any such information (what will you store on your new laptop: just some documents, a large photo or music libraries, tons of movies, ... ?), I am afraid nobody can give a useful advice about a given SSD size here.