"The boy is tall, and the girl is short."
Translation:A fiú magas, a lány meg alacsony.
My solution "A fiú magas és a lány alacsony" should be acceptable. Moreover, the English sentence shouldn't have a comma before "and", that is Hunglish.
This "and" joins two sentences. There are some occasions when a comma is used in this way, but it is rare. The second sentence has to be a new thought strongly connected to the first sentence (hence you don't use a full stop / period). The Oxford or serial comma is for a list of items. That means you need at least three things. Moreover, the Oxford comma is a point of contention amongst UK and US English. You can read more about this here: https://www.theguardian.com/commentisfree/2011/jul/04/oxford-comma-common-sense There is even a song about it: https://www.youtube.com/watch?v=P_i1xk07o4g