It's not too uncommon and depends on the context. Usually you use this to either stress the verb or object which is put in the beginning. Here, it might be: Aunt to now-adult person: 'Gosh, we haven't meet for so long. Und groß bist du geworden!' (you became large, or rather: you have grown a lot [since then]).
Depending on the stress on "Groß" or "du" this has different meanings. "Groß bist du" means "We haven't met for such a long time, what have you grown!" "Groß bist du" means "You call me tall? It's you who is the tall one". "Du bist groß" without particular stress just means "you are tall".
Out of curiosity, do you German native speakers just enjoy doing the lessons, going to the German comment sections, and helping people out, or do you get something out of it, or how does it work? I never thought of going in the comment sections for English/Portuguese, the two languages I'm fluent in.
Either way, danke schön. :)
Well, personally I see it like this: "You get from the community for free, so you should give something back in return of the things you get, as much as possible to show your appreciation. So the community and the things to be shared will be larger and richer". That is why I write blogs since 9 years (technical stuff related to my job/hobby) and reply people's questions at technical forums. By the way, I'm not a native German speaker, I'm a native Turkish speaker and I check out Turkish forums sometimes to look if someone needs any help...
You may have discovered this already, but once you have finished the tree in the language you are trying to learn, it can be very helpful, if it is offered, to take the English tree as if you were a native of the language you are actually learning. The lessons are slightly different and the discussions are in the language you're learning, mostly. For instance, one I finished the tree in Spanish (for English speakers), duo offers English for Spanish speakers, so I started that.
*an 'ss' actually.
If you're on mobile, try holding down the 's' of your on-screen keyboard. On Samsung key board it comes up with ß, §, ś, š, and ş. It should roughly be the same for most keyboards (unless you're using a downloaded keyboard from the Play store or something, then maybe not).
If you're on a computer, Duolingo should have little buttons you can click to insert umlaut characters and essets (ß).